I had a wonderful lunch at Blue Water Grill last month, a chicken Philly (I know there really isn't such a thing). It was about $BZ17.00. I didn't think that was overpriced, came with fries. I would have paid the same here in Iowa.
Same with the awesome chicken salad at Monkey Bites. About $BZ15.00. That's what grilled chicken salads cost around here, and I know lettuce is really expensive on Ambergris.
Same with the fried shrimp at Fidos for lunch. $25.00BZ last time I was there, with fries and coleslaw. Better than most fried shrimp dinners here for less money.
None of the above are cheap places to eat. But, they also aren't any where close to twice as much as here in the States.