Best deal is to get a permit for the emplyee lot, if you can. Get friendly with your carrier/codeshare's station at DTW, the managers can authorize permits if they are so inclined. I had one until about a year ago, I was refunding the station that got it issued for me $10/month. I'm not sure if this was really the cost or if the manager was scamming me, but it was still a great deal.
If that isn't an option, check out Qwik Park on Merriman, up near the crowne plaza and red roof. They have an industry rate that is about $4/day. That seems to be the best around, and the van service is always good, direct to either terminal. Actually, come to think of it, the van service is so much better than the crap employee lots that this might be the best option anyway. Good luck..
..CT