most restaurants at the third street promenade are paying very high rents.
the good quality fish that goes into decent sushi is expensive, even at the wholesale level.
basically what you want, is, imho, impossible to get.
you can get reasonably price sushi at Hide on sawtelle (i.e. not near the promenade).
also, for a little more money you can get a good sushi price/quality/quantity ratio at k-zo in culver city (also not near the promenade)