Midway parking is WAY OVER PRICED, compared to Milwaukee's rates. You can park in the outter lots for less, but the neighborhood down there is not the best. Any time I see fences around gas stations, and bullet-proof glass inside the station where the clerk stands, well, I pay a friend to drive us there and pick us up. Great air line, comfortable seats, lots of leg room, almost all 737's, no assigned seating, etc. Book as FAR in advance as possible for better prices too. I've also heard that there are certain days of the week where prices can be lowered (Tuesday comes to mind). Good Luck!
Park & Fly hotels may work out to be cheaper than parking at the airport. You would have to compare cost based on the length of your trip. Park & Fly's can vary at different times of year. Midway has several; Hampton Inn Chicago Midway, Marriott Chicago Midway, Holiday Inn Midway, Double Tree Alsip are a few.