What is the cost to finish a bonus room per square foot?

There are too many variables involved to give an estimated price. If you live in a house that's under 200k I would start at $30- $40. For a million dollar house I would start at $60-$100+. Just get 3 or more estimates and check references closely. People will tell you anything you want to hear to get your money so know what you are getting into before you close the deal. Best of luck!