FYI All, the OP is not talking about contract work at all. Everyone and their uncle loves to toss around contract rates as a talking point but his situation is clearly not contract.
@gcodeguy
If you can negotiate with them, I would try hard to charge by time, as in time and material, but they sound too cheap for that. I agree with the others that have said bid each job. If you bid it attractively you can keep them happy and potentially have more steady work. Also, as a sole proprietor, you don't have to worry as much as you would paying a partner or staff.