When I was first starting out and I still some times will do this with clients is I would shoot them an hourly rate. This rate was typically 80.00 an hour for 2 guys but I think today it would be closer to 90.
I would tell them I think it will be this many hours and I will not go over that amount. That is just for clean up. Let them know there will be a few hours off the job site to haul off the materials too.