From what I've read/seen, a rule of thumb on a rehab can be a dangerous thing. It can give you a false sense of security and there are so many variables in rehabs (cost of labor/materials in your area vs U.S. avg), sq footage, quality of materials, extent of rehab/potential to discover additional code violations etc as dry wall/flooring are removed, etc. etc.) I would say your best bet is to have another 1-2 contractors and/or inspectors come in to give a quote (tell them upfront you're just getting bids, but make sure you make it worth their time-give them a few hundred dollars for the quote). The extra fees for a couple good detailed, itemized quotes can have the potential to not only give you confidence you're paying fair market value to the contractor, but multiple bids will often give you additional tips/layout possibilities/ideas that you hadn't thought of before and the ability to compare line items between quotes to get a better feel for future projects about how much certain projects actually cost in your area... Use those as your rules of thumb for future projects.