...I would double-check...because usually don't pay more than $200 (usually less) to go in the opposite direction...SW usually is about 75 bucks cheaper from BWI...but CO matches. Dunno about you...but that difference would be made up by the rental and I'd be out the 4 hours.
Also...sign up for US Airways eSavers...if you want to come spur of the moment sometime (I've done it)...those are usually about 140 (same as BWI) and Philly Cleveland is in there at least once a month.