Fifteen bucks isn't much of a discount, especially since they don't have to worry about production, packaging, distribution, shelf space, etc. Plus, most of these games are in the <$10 bins by now anyway. Still, steps in the right direction I guess.
The pricing of the XBL Marketplace stuff is slowly getting out of hand, IMO. $5 for an XBLA game is perfect. $5 for an illegible background theme advertising a game is not. $30 for the Oblivion expansion packs sure as HELL is not.