As others have noted most guides are independent contractors and they have a cost burden which isn't covered by the guide fee, so a tip helps with those costs. I don't do float trips, and generally only do 1/2 day trips so I tip around $50 for the trip. In each case when I've been with a guide I've paid for 1/2 day (4 hours) but spent at least 6+ hours with the guide (including travel time), so I thinka tip is just good form. My rule of thumb would be to tip what you are comfortable with.
"My father was very sure about certain matters pertaining to the universe. To him, all good things - trout as well as eternal salvation - come by grace and grace comes by art and art does not come easy." Norman Maclean, A River Runs Through It