Not quite "cheap", but definitely "reasonably priced" -- Asai, in Belmont Center. The husband and I typically get the sushi special, an appetizer or two, and some extra miso soup (the special comes with one, but we are two :), and it comes to around $30. You can get there by commuter rail if transportation is an issue (or, like us, by bike!).

I used to like Bluefin, but then I started going to Asai, and, well...I can tell why Bluefin's cheaper.