Do it hourly, figure out your costs and bae it upon that, then add another 10-15% to the hourly rate.
Also, make sure your insurance will cover you working as a subcontractor. If you blast out some windows, his isnurance wont cover subcontractors, and yours better cover your tail, or else plan on spending some dough for windows, car paint, etc.
He will most likely have a non compete clause in the contract, if he makes you sign one. Unless your good freinds and respect eachother and know you wont steal eachothers work. But if he does have a non compete, dont make it longer than 2-3 years.
2000 Ford F350 Dump Truck V10 8\'6\" Western
2006 Chevy 2500HD 8ft Western
2005 8x20ft Haulmark Enclosed Trailer
Exmark Rider and WB and Stihl Handhelds