Roof replacement cost

I’m planning to replace my roof within the next 5 years — great timing, I know 🙁 — and was hoping to get some of your experiences, costing information, and roofer recommendations. We’re likely just doing a standard asphalt shingle.

Is cost based just on square footage plus some factor for shape or is there more to it than that? How far apart did you see bids/estimates from different companies? Are smaller companies better value/ higher quality? How did you handle making sure everything was done properly (particularly with respect to water)?

Thanks in advance!