For leisure travel or when my car will be parked for more than a few days, we use Qwik Park, too. The regular daily rate is $8 and sometimes you can find coupons. They are helpful, efficient, and provide good service. You do need to plan for a little extra time because they are not on the airport grounds.
Not sure from where you're traveling to get to DTW, but I wanted to mention that Michigan Flyer has a great bus service from East Lansing and Ann Arbor to DTW. We are going to use that again for our spring break trip. For two of us, it is $80 round trip from East Lansing.