Just got back from a trip last month. We paid (at Jost C&I) $550 for six people for 4 days. This included the national parks fee, and departure tax. I can't give details of how it was computed but I recall thinking that it was a little cheaper than I had estimated.

This was my first time chartering in the VI and going to the BVI. Here are a couple of comments:
1) Workflow at Jost is Customs -> Pay -> Immigration. Officials are helpful.
2) They pre-approved our departure so there was no need to checkout.
3) Every adult needs to fill out an individual immigration form (filling out 6 in the C&I office is a lot of very repetitive work).
4) They need the passports for everyone but only the skipper needs to go to the office.