Rat,
You are right that the cost of a basement is considerable. And if all you do with your basement is store some household items in it, it is very expensive space. However, if you finish your basement, the square footage cost of the additional living space is probably half of the main floors. Maybe more than half, but definately cheaper. Our "wish list" for our home included a game room for the kids, office space for me and my wife, a home theater and, yes, plenty of storage space. The builder estimates the additional cost of extending the basement to the area under the garage will be about $20K which is probably cheaper than it would have cost me to have a free-standing barn built. And I will use that space as a barn - tractor storage and shop.
So, if you have the need for more usable living space, doing it with a basement is actually cheaper than extending the outside dimensions of the home.