Start with estimating how long it will take. Can you cut a tree, carry it out and put it on a trailer/stockpile in 5 minutes? 10 minutes? Say it's 1 tree/5 minutes. 100 trees = 500 minutes = 8.33 hours. So say 1 long day for you and a buddy to cut them and get them to a stockpile location. Will you burn/dispose of them on site or do you need to haul them out? If you have to haul, will the place you haul to charge a disposal fee? Maybe figure another day of hauling depending on how far it is. So, call it 2 days x 8hrs/day x 2 guys x $20/hr = $640. Plus some equipment and tool charge (chainsaws, pickup, trailer etc.) That's a very ROUGH idea of throwing an estimate together. Even better would be if the guy wanting the work done is willing to pay by the hour. Figure out how much you want and go from there. I would think $20-$30 per hour would be fair if it included the tools for the job as well as the labor. But then again, I've never done tree removal