Needless to say, average estimated cost to finish a basement is higher if the project is done by professional contractors. As they charge cost per square foot, the total expenses will be less, if you have a smaller basement and vice versa.n an average, the cost per square foot to finish basement range somewhere from USD 29 - 32 for basic remodeling to as high as USD 50 for installing upgrade features. Do-it-Yourself Basement Finishing.