As most would agree, the ability to port your Stack Of Doom (tm) clear across the world at the cost of 5 mana was game breaking.
Once you had any children with in-combat mana regen, the cost became absolutely trivial and with it the entire concept of movement or garrisoning.
And with 4X games being about growing ones empire, balancing a dominant strategy like teleport on a fixed cost is a recipe for failure.

MOIISKA pointed out one possible solution... which got me started...

Note that all numbers, that are mentioned anywhere here,  are only there for visualisation. I talk about the system here, not any particular number. This is a thread in the Ideas forum without knowing how the game and mana balance will work out in 1.1 it would be completely futile to assign any cost at this point.
(Yes, some seriously wanted to argue about a particular number in here. I kid you not.)

Spell:  Teleport   
   
     should have a base cost of 4.      For that price it only ports the channeler around.

     Every additional unit in your stack adds +2 mana to the cost of the spell.

     Porting a stack of  3 would cost 8 mana, a stack of 12 would cost 26.

Spell:  Teleport Friends

     would have a fixed cost of 10,   plus 0.5 for every additional unit.

     So 16 mana for a full stack.

     With a stack size of 5, either spell costs 12 mana.  Any more units and Teleport group is cheaper.

Travel speed

     Right now Teleport is instant. That is the bit that is entirely too strong.
     You can port to the other side of the continent, have a quick skirmish, and be home for dinner.

     Magical transportation should still have a time component and it should not be entirely predictable.
     I say:   If you cast Teleport, this stack is put in limbo - completely unaccessible by any means - and will appear at the destination 2-4 turns later.
     Much like a MOO2 fleet would be "in hyperspace" for 4 turns and you could not alter it's destination before it arrived.

     Travel time could scale with distance teleported although it should still have a small random element at least.
     Scaling mana cost the same way is probably too messy because you wouldn't find out if you can cast the spell at all until you had selected the destination.

     If you only have one Teleporting Stack Of Doom (tm), then this may actually become your weakness.
     Yes, you teleported towards where the intruders are coming from.  You can probably even kick their butt before they do any harm to your cities.
     But what if this was not the real attack?   If they simply turn around and deny you the open field battle your Stack Of Doom is built for?
     What about the 2 smaller stacks that were sneaking around or used sea transports to stab you in the back?
     The Stack Of Doom would take up to 8 turns to turn around and that can be a very long time.
     A powerful spell / ability / strategy is balanced if sole reliance on that strategy results in a weakness.

Other potential features      in no particular order

• I completely ignored range restrictions here.  They'll have to be put in when balancing the spells because there are too many options.
• Demons of the astral plane.  When you teleport, you effectively travel on the astral plane for however many turns it takes.
  The bigger your army, the higher the chance to be found each turn. And the bigger your army, the bigger the predators it attracts. Tigers don't hunt for flys.
  These... things... would be serious business and able to eat an army. Literally.
• an Evacuate spell.
  A cheaper and "hastily cast" Teleport that sends your units to a random spot in your zone of control.
  Useable both tactically and strategically.    Same travel time rules, possibly even a little longer to make the player think hard about chickening out.
• Different Teleport spells.
  One might be more expensive but slightly faster traveling. And / or you would require x Air shards for that.
  A low level spell should not transport whole armies clear across the world. Possible limit to 3-5 units.
• a chance for Teleport to be inaccurate.
  You might not come out at the exact square you were aiming at.
  The farther you travel, the greater the possible scatter distance from your destination. (a simple problem of aiming = )
  Another feature that would especially hurt the singular Stack Of Doom tactic.
• jet lag / hyperspace sickness
  Units do not arrive in top shape for fighting but suffer negative effects from the teleportation.
  That can be as simple as physical damage but that is easy enough to fix with a Mass Heal spell.
  More interesting possibilities are debuffs that disorient or generally lower the fighting efficiency of the units for 1-2 turns.
  Number of Air shards controlled would be a perfect way to lower or eliminate these effects.
  This would not affect "peaceful" use of the spell but severely limit the ability to air drop a Stack Of Doom onto the enemy's head.
  It would make Teleport a transportation spell instead of an attack spell.
• Teleport transports faster, up to near instanteous (start of the next turn), when starting point and/or destination are a (air?) shard.
  High level cities can build "gate" improvements that have the same effect.
  Personally I strongly disagree with any way to make Teleport instant again but the idea came up...
• Some kind of general spell to detect Teleporting armies in transit.  Astral Awareness?
  That would be the base for all the suggested interception mechanics.
• Possible downside to Teleport, applicable to all teleports or only to those allowes to teleport into enemy territory:
  The teleport spell - while powerful indeed - leaves a visible trace on the strategic map. This translates into a series of small scale Earth Lore spells (1 turn duration), allowing every player in the game to see the area.
  Coupled with teleport travel taking several turns, the enemy would have a chance to prepare and you would pay with intelligence about your own empire if you overused it.
• A teleported stack does not arrive as a consistent army. Instead, one unit per turn arrives at the destination.
  That means that initially, a quite vulnerable caster arrives, followed by a pretty random troop composition.
  Teleporting a full stack would take 12 turns to arrive. While still useful on a strategic scale because it could be beneficial to arrive there in "only 12 turns", it would prevent the cheesy tactic of teleporting and attacking at full strength righ away.
  The obvious consequence: Players use 3 champions to teleport  1/3 of the Stack of Doom each. That, however, triples the supposedly substantial cost of teleporting and still makes your arriving forces somewhat vulnerable with the champions arriving first. IMO, a massive mana expenditure should be allowed to have an unbalancing effect - unless it is regularly used as a dominant strategy again. Then the system would have to change, maybe making teleport a global spell which you could only cast one of. That way, no 3x simultaneous teleport.

Elemental Teleport Variants       after all, Elemental is the name of the game...

• Airwalk     Units can teleport to any place. Units traveling like that are visible to the enemy at long ranges and vulnerable to strategic spells
  like Hurricane, fiery rains, blizzard, metor showers... pretty much an exposed position without any cover.
  Current wind direction and speed (displayed on the map and can be altered with magic) would affect  Airwalk,  so you might need
  more than just the Airwalk spell to grant your travelers favourable winds...
   
  
• Waterwalk    Units can either teleport from coastal square to a connected coastal square or it could simply be a 1-3 turn buff to let all units
  in the stack walk on water at triple speed or something like that.
  Effectively the same vulnerabilities as Airwalk.
   
  
• Earthwalk    You can travel between "earthly" resources like quarries, clay pits, or mines.
  The travelers would be vulnerable to earthquakes.
   
  
• Firewalk    Travel from hearth to hearth (city to city).   Simple but boring.
   
  Alternative:
  You literally travel through... fires. We're talking big ones on a strategic scale.
  This assumes the ability to create fires on that scale. A strategic fire spell to create a 1-square fire that lasts 1-3 turns.
  This fire might spread (or only move) through forests or cities. (see "wind" with Airwalk)
  This in itself is one of the drawbacks of the Fire Port. It can go anywhere but the enemy channeler would have a pretty good idea
  on where you are going. Hint: look for spontaneous and huge columns of fire in your empire.
  Creating the "entry point" on your own territory might actually create more fire than you bargained for. Maybe the enemy is a Wind channeler and
  creates a storm that blows your fire into one of your forests or cities, letting it spread through your empire...
  The travelers travel in the shape of flames so the travelers themselves would have a very low chance to create fires along their path.
  They would only be vulnerable to water spells.
  Maybe the destination fire can be extinguished with water spells, letting your traveling army "drop out of hyperspace" and take damage / debuff.
  There must always be exploitable weaknesses when allowing "assaults" via teleport at all.
  They could also sometimes end up directly in the destination fire so asbestos underwear is recommended with Fire Airlines.
  
  You'll note a lot of fiery side effects here. Fire is not particularly well suited for such peaceful applications. Sure, usually it will work just fine but
  worst case you cut a path of destruction through your own empire only to have your army end up in the magical fire you created yourself.
  Fire is fun to play with but kinda volatile. =P

The quickly increasing cost of the low level Teleport is actually a good thing.
It gives us a reason to pick the Air book (which costs reasonable pick points now!) and research Teleport Friends.

Teleport Friends would eventually take over once your empire and your army grows.
Teleport can remain the early and general spell and an expensive fall-back for players without the Air book.

Sometimes you absolutely have to teleport, regardless of having the group port or not.  Regardless of the cost.
That should not immediately be a game over situation (No, you cannot teleport that army!) but merely put a painful dent in your mana supply so it turns into a temporary setback.
That creates another option to think about. Another case of pricey but worth it.

Of course, I don't know if the numbers work out like this.  This is only a draft for a system that could work.
Maybe Teleport is still too good and needs to cost +2.5 or +3 mana for every passenger unit. Until we know the mana situation in 1.1, it's all guesswork.

Related:   [Suggestion] Travel takes too long, add Gate spells.