I'm sure in some places DMs get paid something for charters, or possibly room and board & tips. Many places (U.S. a lot) DMs assist classes with no pay or tips-- that one is puzzling.
As a DM I got paid for guiding dives in Egypt - €12 per dive, usually 2 dives per day.
In the UK I assisted with classes with no pay or tips (though I rarely bought a beer in the bar afterwards). I even paid my own entry fee to inland sites sometimes.