Revealing a military secret, I'll say that when... how long ago was it? 5 years ago, when I was making this mod, the idea of making the hiring prices for the two upgrades different was indeed present. But obstacles that arose along the way prevented its implementation. The problem was this: it's possible to make the prices different (which would add even more variety to the game), but this raises a serious issue when upgrading creatures from one upgrade to another. In the original game, such an action costs a very small amount of gold. Here, however, it would be completely different. For example: one red dragon costs 3 sulfur and 3 crystals to hire, and then you decide to upgrade it to a black dragon, which costs, say, 6 mercury. Do you think you'll just pay the difference of 6 mercury for this? (Actually, you won't, but more on that later). And will anyone return the 6 units of other resources that were previously spent on the red dragon? No, they won't. In the end, you'll pay twice (yes, it's similar to how Russian banks work with loans: you take out a loan for an apartment, and after 10 years, you'll pay for it as if it were two apartments). The same "flaw" will occur if one of the upgrades requires only gold. It will be IMPOSSIBLE to upgrade a "resource-based" upgrade to a "gold-based" one without losing what was previously spent. And that's not all. This same "trick" with upgrades can be used as a bug for selfish purposes. In other words: you buy creatures at their original cost (in gold), and then upgrade them to "resource-based" versions, for which no resources are charged. It's a clever trick, but it can't be fixed by the mod, because I realized that such things are already deeply embedded in the game's code, and ordinary mortals have limited access to it.