It can vary widely depending on if you're doing the work, a handyman, or a general contractor. Further, finishes can vary the price dramatically as well. If done professionally, you could easily spend $30 per sq. foot on the livable areas... you could get away cheaper on the store room...
Things I think are important to keep in mind:
Ceiling height around ducts & plumbing
Making sure you have enough hvac & electrical
Natural lighting (i.e. an egress window or two)
Maybe another bath (depending on if you have 1 or 2 already)