Roofing estimates to replace flat roof?

We are looking to replace our flat roof, as it is about 30 years old. It is currently tar and gravel, above the second story.  It is pretty simple - no skylights and in pretty good shape.  No major wood rot or leaks that we know of.  Does anyone have any estimates or costs they actually paid, and that they'd be willing to share, on a roof replacement in the last 5 years, being aware of course that costs vary in each case?