Do monsters drop gold? I've killed a few but they only give exp.

Which game?

Not all creatures drop gold. It always bothered me in other games when a wolf would drop gold or something silly like that. Why does a wolf have gold? M&M does it right, as far as I'm concerned.
Most humanoid monsters carry gold, or items that can be sold for gold, but by far the best way to make money is to complete quests. People are very rewarding sometimes. Also chests often contain gold. These things hold true for all the games.

Amen to that! :)

If you're in one of the first couple games you'll have to search the square you are on after you kill the monsters and before you move off the square to get gold

And if you're in a bad way, you can stop to sleep (or at least heal your party) - just don't take a single step forward, or something else will take it away! Spin in circles, cast up your beneficial spells, rest for eight hours, just don't step forward or back.

In MM1 and MM2, press "S" after combat, then "Disarm Trap" using a robber or ninja character. Make sure that none of your party are unconscious before you do this, because if you set off a trap it will kill them. Make sure you press "S" before you move away from the square where you had the combat, or you'll miss out.

In MM6, click the corpses of dead foes.

In MM3-5, you get gold automatically when there are no more enemies visible.

Not all monsters have gold or treasure, but most do.