You ask for a place in Grand Case, right? Why not stop in Marigot on your way. Near the ferry dock is a place called Rosemary's (near the walkway to the market sellers) whole meal of snapper, rice, salad for around $14. Very filling, tasty and inexpensive.

I mentioned La Villa because it isn't as expensive as the other places in GC. I would suggest NICE on the water side but I'm not sure when they open up. They're less $$ than other places.