There are basically two charter companies operating in Belize: the Moorings, with a few boats in Placencia, and TMM with boats in San Pedro (Ambergris Caye) and Placencia. Of the two, you are likely to get a better deal with TMM, which has a larger fleet with a greater range of ages.
However, a couple of things conspire to make Belize more costly than the BVI. For starters
, you have only two charter companies -- no competition=no deals. Secondly, without a huge marine industry, it''s much harder and more costly to maintain and repair
the boats. Provisioning and other supplies are harder to come by, and more expensive, as well.
I chartered with TMM in Belize last February. It was probably the best charter I ever had -- and I''ve done over a dozen over the years. The solitude and escape from civilization (and hordes of other charterers) was priceless, rendering the cost virtually meaningless.