Iirc, the roast crab is seasonally priced. The times I went there, it was between $28-32, and is enough for 2, but that depends on how much you eat. 1 roast crab and 2 portions of garlic noodle (~US$8-10 each) plus one appetizer is usually more than enough for 2 persons, imho. Appetizers run from US$8 to 20(?) I think.
Like others said, Crustacean and Thanh Long are related, Thanh Long being established earlier. The SF Crustacean and Thanh Long seem to have very similar menu, with slight difference in price. The Crustacean in Beverly Hills though, is a different story.
It's obvious, but make sure you get reservations and arrive on time. They could bump you to a later time slot if you arrive late to accomodate for the people waiting in line.