I think a fair reward from a monster is (average cost of a force with a 85% chance to beat the monster) * 1.25 ~ 1.50.  Play well and you can do it with a weaker force, making the return better (well, once tac combat is challenging).  Steamroll it and you've gotten (some of) your money back from buying the force required to kill it, but in exchange you risked your stack, spent time training, spent time moving, spent materials, etc.

Example:  If you want to go kill a dragon that would typically require an army costing 400 gold to give a pretty good chance at a win, the dragon should drop 500-600 gold.  If you manage to do it with an army that cost 10 gold, you still get 500-600.

And there needs to be taxing, yes.