I got a soft case made by Uncle Mikes that works great with M&P 15-22 (and also with AR 15, of course). It has 5 mag pouches, a large outside pocket, good handles and a sling strap, all for $28.99 from OpticsPlanet.com:
Uncle Mike's Law Enforcement Tactical rifle case. 52121, 52141. Uncle Mike's Gun Storage.
Anybody seen a better value?