Now that you have your estimates, it would be OK to go back to each of the estimators and ask some questions.
Tell the $2500 contractor that you've got an estimate for $5500 to do the driveway as specified. Why should is yours so cheap? Then, go to the $5500 contractor and tell him you got an estimate for $5500. Why should you pay so much? And do a similiar 'thing' with the ones inbetween. Hopefully, you're getting the estimates in writing that SPELL OUT what you're paying for. Among other things, the estimate should include the driveway dimensions, the concrete's PSI, location (or number) of expansion joints, excavation (if necessary), back-fill (if necessary) for 'raising' the driveway to drain AWAY from the house, the concrete's thickness, and estimated start and completion dates. Try and have the driveway done in the Fall when it's cooler. In the summer, it's possible for the driveway to dry too fast. If it must be done in the Summer, ask the contractor what you should do to keep it from curing too fast. (Usually, you wet it and cover it w/plastic, or just wet it a lot in the morning.)
Maybe other folks have some more insight. Go ahead and play one vs. the other. BUT be sure to get your estimate in writing! My best to ya and hope it helps.
PS: COnsider widening your driveway a foot or more. You'll LOVE the extra room. Talk to your contractor. Maybe you can 'dig out' the extra foot or so to 'the proper depth' to save some $$$ on the extra driveway. This way, the contractor only has to lay in the gravel and concrete.