Average Percentage Increase from Proposal to Final Cost

I’ve had a few contractors look at my condo’s roof and have a few estimates. A couple state that their prices will increase depending on how many tiles and how much wood needs replacing. I understand nobody will know how much actual work needs to gt done until they actually tear up the roof, but in order to pitch any contractor to my condo association I’d like to get some sort of reliable number.

I’d REALLY appreciate if you could give an idea of the average amount a total roof replacement goes up by. It’s a 15 unit building. Thank you very much.