I went to Belize this time last year and I'm headed to Roatan in less than a week. From what I can see, air fare is actually cheaper to Belize, but accomodations were more. We stayed on Ambergris Caye at the Belizian Shores Resort and Tuff E Nuff Tours was right at their dock. Seems to me that it was about $350 a day for the two of us to stay there and for a two tank boat dive. They did not have an all inclusive price that included food. So we probably spent about $50 a day at the pool bar and having lunch there. Then dinners were running close to $100 a night. I don't think we had any alcohol with those dinners either. We never did eat dinner at our resort. We went down the beach to Capt. Morgan's, up the beach to the Journey's End, and to San Pedro a couple of nights. So IMHO, food and drinks are expensive, lodging and diving was reasonable. Oh, and going to the grocery store didn't save much money ($18 for Boone's Farm, ouch!). The big one is just south of the Tropic Air terminal at the San Pedro airport. So if you want to buy some stuff, the time to go is when you arrive.
We liked staying up on the northside of the island. It was nice and quiet and we really didn't see very many kids. We missed the water taxi to town one night, so instead of waiting an hour, we just walked to town. I think it took about an hour to make the trip, so we didn't save any time, but at least we got some exercise. One of the nicest things about Ambergris Caye was that there was nobody on the street looking for handouts. We did see a few merchants along the boardwalk, but they were not high pressure.
If I had it to do again, I would stay at Captain Morgan's or the Journey's End and dive with Tuff E Nuff. It might be a little more expensive that way, but diving with Billy and Tuff E Nuff would be well worth a couple of extra bucks.