When I was waiting for the Airport Fast Park shuttle at BWI recently, I saw the shuttles for the new "EconoPark" private lot go by.

EconoPark is farther from the terminal than even Airport Fast Park's Blue Lot, but the price is right: only $4.75 per day with Internet coupon. I haven't used EconoPark yet, however, so I don't know how their service compares to Fast Park.

Here's the link:

http://www.econoparkexpress.com/onlinecoupon.html