While in my local Walmart (Elberton, GA, 30635), I noticed that they had the Starcraft 2 Battlechest and the Diablo 3 Battlechest clearanced to $6.00 each. This is a highly YMMV deal.
I know that the base game for Starcraft 2 is going Free-to-Play in a few days but for those interested in obtaining the expansions cheaply, this could be the best way to do so.
Starcraft 2 Battle chest [brickseek.com]
Diablo 3 Battlechest [brickseek.com]