I have used Nanny Cay for the past 3 years. The fees depend on several factors which change each year. Here is the Web link
Nanny Cay Resort, Marina & Boatyard - British Virgin Islands - Home
FYI, this is actually the first year I have seen the Marina not totally full or booked out early in the year. In Nanny Cay, several of the charter operators have cut back on slips and storage requirements and an excess has developed. Due to the over supply of charter boats and the different tiers of owner compensation, many owners of the charter fleet boats have removed them from charter and I know of a half dozen or more that have returned the boats to the US until/ if the market in the islands changes.
Several in water storage locations are available for about $12 to $14 per foot for real boats (mono's) and everyone charges a premium for the trailer's on pontoons.... ;^P
Which you use will effect your insurance cost but they all generally have "approved" hurricane procedures.
I can say it is really nice when you can have your boat splashed after an On the Hard storage and sleep aboard the day you arrive by air/ ferry and be gone sailing the next AM.
Nanny Cay does have the best showers/ baths in the Caribbean!!