You need to get several bids from reputable contractors in your area to determine the cost. Look at the selling price/square foot in your neighbood and then times that by the added square footage to determine if it is economically a good idea. It usually is!

Tony - It would all depend on the amount of square footage you are working with and types of finishing you have in mind. I have referenced a great article tht can be found at http://www.realtor.org/rmomag.NFS/pages/2007/CostvsValue?OpenDocument that may help answer this question. If you have trouble opening, email me and I can send you a PDF version of this. Good Luck!