The fair price per sq.ft. would mostly depend on location and condition. However with the current inventory and market condition that price will generally be lower than it should be. You are looking anywhere from $15-$25 per sq ft. but again, that number is subjected to many, many factors. Don't let those numbers throw you off!
To really answer your question correctly, more info is needed from you on exactly what you are looking to do and the location...your needs will better narrow the price.
I would be happy to help, just give me a call or Email me with what exactly you want.