Improved balance.

Nargott has very accurately described the essence of the problem; no further explanation is needed – I agree. Heroes is not Civilization; it's a combat strategy game, not an economic one.
Where do you get the money from if you start the game with absolutely no resources (on the highest difficulty)? Or do you pay the deceased troops posthumously the next day? And another problem arises – sometimes, to complete a scenario, you need to assemble a strong army. Because of the troop upkeep, the time it takes to assemble such an army can increase several times – it's meaningless, routine work, not a dynamic game.
Regarding resources – have you considered that, for example, mercury is not needed to build a building, but for some kind of bath in which a newborn monster is bathed so that it acquires special abilities? Use your imagination.