My advice to you is call a local agent and have them send you neighborhood comparables. There are so many factors from list price, condition of the property, interior amenities, etc that could be much more important than a deck. You may find its better to spend the money elsewhere in the home, or it may be easier to adjust the sales price and keep your money in your pocket. With a deck you will recoup some but not all, of the money spent. Good luck!!