Christa, we added a basement to our home and it turned out wonderfully. Without knowing previously, nobody would really guess now to look at our home it was anyhthing other than a stick built.
We were very fortunate to find a mason and a excavator who were able to work together well. The mason had plans on hand for a home just like ours.
The problem that manny pointed out is valid. Is there a place inside to access the basement? We ended up having to have stairs down into the lower level from the existing master bedroom. While within a few months when it's converted to a formal dining room it won't be a problem, it is a headache now. Because of the steel beams and the added steel ibeams welded to the existing steel beams (mortared into the masonary walls) there was not a way to put the stairs in a better location.
We added a media room, half bath, laundry room, guest bedroom and master suite. We're working on the master suite now. Hopefully it will be complete by the end of summer.
Whatever you *think* it's going to cost though, it's probably going to be double. Our 36k loan paid for the excavation, walls and waterproofing, steel beams and some plumbing. We probably have put another 15k (at least) out of our regular household budget and credit cards in the past 9 months on other stuff like drywall, wiring, flooring, fixtures, lighting, doors, etc. We are lucky not to live in a major metro area. I'm sure we would have had to pay three times that somewhere else.
Make sure that you plan on living in the home for a LONG long time. The cost of the improvement may end up in the short term making it impossible to sell at anything but a loss. Also, have you discussed this with your lender? They will send out an appraiser to judge whether the cost will be supported and if they'll loan you the money.
Here's a link to pictures of what we had done a month or two ago, the master suite is coming along nicely since then though :~)