I would like to recommend two Sushi restaurants. Neither are cheap but then I've never found reasonable sushi prices here in Zurich!

Sala of Tokyo is located just opposite the ABC cinema in Zurich, by the main Station. It is very small and has funny opening hours. Reservations are definatly recommended. I've often wanted to eat there, only to find that its all booked. But the Sushi is very good!

The second sushi restaurant is Okara on Brandschenkestr. 5, 8002 Zurich. Very simple decor, nice food and they have a happy hour during which a Bento box costs about 40.- This is a very generous selection of various Japanese dishes.