If you can walk at a normal pace and deplane quickly (not seated in rear of aircraft and don't need to wait for a valet checked bag), I think 30 minutes is generally doable at either airport but it doesn't leave time to spare assuming that both flights are mainline or large RJ (and therefore unlikely to use gates in A/B at MSP or B/C at DTW. However. two caveats: for this, you should (a) be an experienced traveler and generally know the relevant airport layouts and (b) have figured out Plan B and Plan C in case you do miss the connection and determined that the risk and consequences are acceptable compared to the schedule convenience and time savings associated with the short connection. I would be reluctant to take these short connections just to save money because if there's a big price difference compared to flights with longer connections, I would be very suspicious that those alternative flights are already very full.