We have run into the same problem. We came to the conclusion that a subfloor would be a waste of money and since we had only 7 feet from the floor to the joists we would lose headroom. When you put a subfloor what you are doing is preventing the moisture from coming through and that if not done properly can cause mold and then you will be looking at replacing it. What we did was put the carpet right on the floor. This allows for the moisture to flow right through. It has a special underpad that is almost an inch thick. The underpad is made for basement floors. The carpet it's self is a berber. The floor is very soft and warm. The room is about 300 square feet and it costed about three thousand dollars CDN. If you decide to do this you should also think of putting a dehumidifier down there to prevent the carpet from smelling if you have a moist basement. If you live in the United States or Canada you can ask the people at The Home Depot.