From what I understand, $48 is pretty much wholesale cost of video games for retailers. The idea with Best Buy is to counter the lack of profit with money earned via membership fees and repeat customers. It encourages a gamer to shop with them since they will typically have the best deal, and while in the store may wind up buying other items, too. Then there are the folks who refuse to get a membership and will just pay full price each time.
All in all I imagine this has been a huge success for Best Buy, or else other retailers would not be copying them or otherwise matching them with various deals.