According to an article in Wall Street Journal Activision has spent "more than $100 million" on Starcraft II development. A princely sum that puts the game up there with the most expensive games of all time, and not unreasonable given the prolonged development time. However, the article does not cite any sources so it may only be an estimation.
The first part of the Starcraft II trilogy - Starcraft II: Wings of Liberty launches worldwide on July 27 and some analysts expect it to go on and sell 5 million copies in its first year.