A crystal ball may come in handy on this one.....
Out buildings definitely add value to a property but the amount may depend on a number of factors including the size of the lot and possible utility for the property. It's difficult to say what your future buyer's needs will be.....Keep in mind that you could go throught the cost and effort of erecting the structure only to find that the buyer will take it down because it doesn't meet their needs.
Another way of viewing this is that by putting up the building and increasing the price, you could be pricing potential buyers out of the picture. Knowing what the demand is for these types of opportunities in your area would be key to making a good decision.
My feeling is that you can't go wrong by doing nothing.