I fly frequently out of BWI and I've always had good experiences with the on-site economy parking. I think $8 is what I paid the last time I went and the shuttles are very frequent and drop you off not far from your vechicle. The next tier of parking is in a covered garage and I think that is like $10-11 per day. Both are cheaper than when we fly out of Dulles or National.
Good Luck!