Again, the single player game isn't any shorter than the original Starcraft, it's just all done with one race. I doubt the 2 expansions will be $60 each, although I wouldn't expect them to be less than $40 either.
Blizzard hasn't put out a bad game yet. They've killed at LEAST 2 near-finished (one was completely finished) projects because they weren't pleased with how they turned out. If any company can sell their games on faith alone, it's Blizzard.
I'm not only excited to play the SP game and some versus (although I'll admit I'm not a HUGE fan of the versus), but I can't WAIT to see the crazy mods people come up with.
edited to make the 2nd paragraph seem less ADHD.