In time for the traditional “Black Friday” sales we see in the US the day after Thanksgiving, the Blizzard Store has cut prices for digital purchases of most of their titles this week. You can now get Diablo III for $40 and Starcraft II for $20, plus a discount on older WoW expansions.
You presumably own a copy of Diablo III already, but you might have some friends who do not? Or you can just buy some extras to expand your gold farming operation more easily than swiping them from WalMart.