I can't imagine it being worth the money. The chances are that you would have to put in a completely new foundation, so that it went down deep enough for your room. As soon as you start digging down below your current foundation, which may or may not have a proper footing, you are undermining what the house sits on. It really shouldn't be done as the whole foundation could fall in. You would probably have to jack the whole house up on i-beams, remove the existing foundation walls, dig down 8 or 9 feet, pour footings, pour or block new foundation walls, pour slab, along with insulation, drainage, footings for bearing points, related engineering, etc. etc. You could easily spend $20,000 (probably significantly more) just to end up with an unfinished basement under a cape. There is no way that you could ever make that back with the sale price, as basement improvements don't count for much toward resale value. I think you would be much happier and more cost effective to put an addition on your house or raise the roof or dormer it for a second floor. This be more likely to add AT LEAST what you spend on it to the resale value of your house, probably cost less per square foot of usable space, and give you a nicer space than a basement.