There are literally less than 20 Games on Demand titles and every single one of them can be purchased at retail outlets
Every single one is more expensive than retail, too.
There's a reason for that, the reason being that sales come almost solely from B&M and the last thing Microsoft wants to do is violate any agreements, written or simply understood, that has the potential to take some of their shelf space away.
This is the same reason why Episode 2 was cheaper in stores than on Steam.