Depends on your setting and campaign. If you're fighting draconic cultists who are amassing wealth to fund a big bad project, then you might grab some of that wealth as you defeat cultists. If negative energy from the Shadowfell is leaking into a forest, causing woodland animals to rise as undead and attack nearby farmers, and you destroy those undead and close the leak, then you have the gratitude of the farmers, the Emerald Enclave and the Lords Alliance (dead farmers = no tax revenue). But those skeletal elk don't carry gold coins, nor weapons you can scavenge and sell. When you destroy a skeletelk, you don't even harvest edible meat!
If you want guidelines, you might look at fantasy literature, and assess the income-by-level of Beowulf, Odysseus, Legolas, Sparrowhawk, Conan, King Arthur, etc.