Labor rate + disposal fee + extra profit to stick in your pocket.
Don't do it if your not hitting a homerun. If you dump for free still charge them a dumping fee. They need it gone, you need $$$$, and you have other jobs that could be getting done. If you find trash, rocks, or large sticks, toss them back into their yard. Keep your eyes open. It's a leaf vac not a wood chipper.