I checked the "snacks" sticky (top of the board) - sushi is not listed at the Poly. I think it's mostly things that cost $4 or less, was the last dollar figure I heard thrown around.
The cheapest sushi at the Kona Sushi bar is $8.99 from the allears menu and it's dated last summer.
However, I did see sushi on the Capt. Cook's menu, so you were right about that. But it is also listed for $8.99.