I'd wager the scale they cook at reduces the cost per meal significantly.
Although, I was at a sushi buffet (the kind where you order it and they bring it to the table a few minutes later) once where my friends and I ate so much sushi, they started slowing down our sushi "refills". Eventually we got the hint and left.
Is the eating a cost a traditional bell curve? At what point do buffet's lose money?