1 What do you mean by "efficiency" of a room?
2 How do you distribute to what creatures you want the imp to deliver? Do you think it will work better than throwing creatures directly? How do you control the imp considering upgrades? what if the imp takes the armor you're about to upgrade?
3 So what your saying, that 10 goblins are eventually, worth 1 bile demon? that is ridiculous, do you want to invest in 10 weak creatures or 1 powerful?
You totally missed my point. Assume that your making an upgrade for goblin (armor). You see a goblin icon, choose armor and see the price. Thats all. Simple. You choose another creature, set the upgrade, see the price. below there will be arrows <> to increase/decrease number of upgrades to be made.
I disagree with the library upgrading. How do you make warlocks to research an enchant? How do you differentiate? It's much easier to throw them into workshop and make them do some magic over the armor.
"Do you think the percentage increases are too high, or too low? Consider there is one upgrade only yet it can be used for different creatures (Like 3 "fire" goblins and 2 "ice" goblins). Also consider that the real bonuses go to magic users and magic creatures who usually have low physical defenses and can be taken out by probably one hit of a giant."
Strategy. Who needs magic upgrades when you face mostly melee units. It's like Red Alert or any other strategy. Why buying airborne units when enemy has a lot of anti air? This is the key. The more variations of gameplay, the more interesting the game.
I am also thinking of making "either or" option. you have either an armor upgrade, or an enchant upgrade. This would make you think more and analyze the situation.