We did our own for $2500, but we did all the work! I would get estimates (free) and let each one know that you have other bids. I would also do research before I get these bids so I know what shingles, tar paper, vents, general materials would cost, so you know if they are quoting materials too high? I just watched a peoples court case where the roofer ordered 38 bundles of shingles with her money and they job only needed 11, and he returned the remaining bundles and pocketed the money when he returned them? Maybe you can purchase materials yourself and the bid would only cover labor?
We are getting ready to gut and rebuild our master bath, but we do all our own work, its not that hard to research the details!