My friends, let's not forget that training soldiers is a complex and time-consuming process. I used to wonder, "Why do dragons need mercury, and knights need crystals?" But compare the game to real life: we buy bread in a store for paper money and electronics for electronic money. The same applies to the world of Heroes. Crystals are used to pay for the work of builders, architects, construction material delivery personnel, major repairs, and hot water; they are exchanged for other types of resources (food, sand, and cement, for example). It is quite possible that the Academy of Magic uses guest workers from the Elven faction (or vice versa). Think bigger. Be glad that instead of all these complications, we simply give twenty crystals and get the building we need. Consider it the magic of the Heroes world.