This question comes up a lot... I'm sure there are people who use these $/SF estimates and have success with them. The truth is other than individual items/scopes on a larger project, there is a good chance putting a standard $xx/SF to rehab will end in over or under bidding the job, probably by a large margin.
When I worked for a mid sized GC (commercial or extremely high end residential) we "kept track" of $/SF of each job. This was only done after the project and had zero bearing on any future bidding. It was only really to look back and say "wow, we spent $200/SF on that house".
A cosmetic rehab on a 10 year old 800 sf condo with low-end finishes and work done by a handyman is going to cost much less square foot as a gut-rehab of a 100 year old 3000 sf Victorian with high-end finishes and work done by artisan contractors and a high-end GC?
Cost per square foot is meaningless. Without knowing the type of house, the age, the scope of work, the level of finishes, the type of contractors, your negotiating skill, the time of year, etc., nobody is going to be able to give you a $/sf average.