I'm only 20 minutes from Mitchell Field and fly out of there all the time. Airtran has always been the cheapest for me to fly. I usually get a non-stop that gets my into Ft. Lauderdale by 11:30 am. There's not a problem with crime in the area of the airport, so I wouldn't worry about leaving your car there.
Frontier Air Lines has cut flights out of Milwaukee, so there's less competition which will bring prices up. The biggest culprit is the price of oil. It's still cheaper than driving if there is only two of you.