I highly doubt Diablo 3 has been the cash cow they had hoped for, and with the first Starcraft expansion launched with no sign of a monetization route with custom games I can't imagine their ROI on it is anything breath-taking, given the developmental resources required for a game as widely played as SC2. All of this is set the back-drop of a rapidly waning WoW playerbase and revenue.

Blizzard is the exception to the rule in that they typically deliver very high quality games, actually making enough money to fund that level of quality has not been consistent, while making strategic moves toward more money has consistently ended up burning them (WoW paid pets and mounts showed up about the time the player base started to get bored and diffuse to other things, the D3 RMAH has been an utter disaster since it launched)

Diablo 3 has sold over 12 million copies on PC. That's an extremely high sales number for a single platform. Also, the RMAH has been a success as far as I can tell. I don't have access to any major statistics, but anecdotally speaking I know several people who make a nice profit in the RMAH, and Blizzard takes a juicy cut all the time.

I think they rely heavily on their sterling brand for that consistency. Diablo 3 and Starcraft 2 have done very well commercially, but most of the enthusiasts I talked to were fairly disappointed, and some of them even remarked that Blizzard has left their "autobuy" list. They were one of maybe two developers on that list for me, but I think that explains their very consistently massive success - they used to turn out games that were consistently much better than the competition's, especially in terms of replay value, and so they got on that list for a lot of people.