We charge about $100 for a full clip. It's a lot of work, lots of time, and wear and tear on your clippers! PLUS oil and coolube, etc. Make sure it's worth your while!
Time and wear and tear on your clippers - and your body.
I would also have them sign a release. Horses can move at the wrong time and WHOOPS there goes a chunk of forelock - or hide. You do not need to be 3/4 done and have them decide not to pay you for some reason!