Not sure where you live, but here, negotiating with a roofer is near impossible! It's not like working with a carpenter or taper where they will likely get the next job from you. Afterall a roof lasts for years!
I will also share with you that the previous owners of our home "skimped" on the roof...they only put up a 15 year roof (which really means about 12 real years) and it showed! We just put up a 35 year architectural roof and our house looks so much better -- even our neighbors have commented on it. BTW, our "15" year roof was the second layer so we did a total tear off -- and the "15" year roof, we removed it after 10 years -- not because it leaked, but because it looked awful with nail pops, fungus, etc.
My advice would be to cut corners elsewhere -- but not your roof! Also, if all the bids are coming in near one another, then you are likely getting good quotes. We got 6 bids when we did our roof. 1 bid was $13,000. 1 bid was $4,500 and the rest were in the $8,500 range. We selected one of the 4 contractors who was in the $8,500 range and based it on recommendations from friends and neighbors, checked him with the BBB, and a few other contractors we know. We insisted on a certificate of insurance that named us as additional insured (in case someone fell off our roof...it's steep) and agreed that we would pay him in full on completion and agreed that he would provide us with a lien waiver (it's due to weirdness in MO law).
BTW, the guy who bid $4,500 and $13,000 wanted 75% up front -- NO WAY! I would have paid for materials upon delivery, but no way would I pay for anything else before the job was done to my satisfaction.
Good luck with the home renovations!
BTW -- we have successfully negotiated prices with carpenters, tapers and an electrician (we needed some extensive work there and he gave us a break on a new box).
K