The Vine Ogre

[Oversway]

Another approach would be to keep a higher price but make the drakes/wyvern better. Zen's mod was more to this approach. I don't know if it was effective or not.

At low spell levels I'll usually want my mages researching instead of summoning. It is usually not much more research to get something that (at least appears) better. Maybe with very difficult research you might buy drakes?

[Agrajag]

My unit value calculation will probably be:
(HP/10)*(Morale/10)*(Protection/10)*(Strength/10)*(MR/10)*(Attack/10)*(Defence/10)*(Precision**/10)*(AP/10) = Unit Value.
If you want to include Encumberance just divide by it (but Encumberance isnt always important).

** Only if important to that unit, otherwise just ignore Precision