Most of my cleanups are for my regular lawn care customers. After October 15th, all mowing and cleanups are billed at my hourly rate. I just keep going through my list and hit them all 2 or 3 times. I don't give them any sort of estimate, I just do it and bill them at the end of the month or completion of their final cleanup, whichever comes first. I've never had a complaint doing it that way.
Fall cleanups can be tricky to estimate and the effort required can vary quite a bit year to year. It depends on whether it's wet or dry, the rate at which the leaves come down, whether there are storms that litter the lawn with sticks and twigs, the amount of acorns dropped and whether the wind has blown the leaves from or into the neighbors yards. Ever try cleaning up frozen leaves? That's a ton of fun, lol.
For most one-time cleanup customers I give them an estimate along with my hourly rate and disposal fees. If it's a fairly small job that I know I can estimate accurately or I know it will fall within my minimum fee ($150), I will give a lump sum price, but I don't do many of those.
I fully understand what the others are saying about not working by the hour, but it works for me. My customers know and trust me and know that I work my tail off and that they'll end up getting a good deal.