The things that we found to be over-priced are imported goods - snack food, etc. We ate mostly cereal, pastries, sandwiches and fruit while we were there so I can't really comment on pastas and olive oil.
I didn't personally take the ferry from Belize City to AC, but three of my friends took it and were not to enthused with the experience. Apparently the boat is very crowded and very windy. My husband and I took the flight and loved it. It is a quick flight with an amazing view of the island. Plus, we spent enough time on boats (snorkeling, tours, etc) while we were there.
Just my two cents.