Most charterers prefer to use mooring balls, which will run to $25 per night (it is recommended to dive on these to make sure that they haven't been chewed up by props). Some anchorages have little or no room for anchoring (The Bight - unless you have enough chain to anchor in 50-60 feet, which no charter boat does) and others don't have great holding (Great Harbour on Jost van Dyke, which now has mooring balls).
Some places ( The Bight and Bitter End come to mind ) have raised their prices to $30 for mooring balls. Sprat Bay charges $60 I believe - I have not taken them up on it. "Moor Seacure" seems to do a pretty good job maintaining moorings - I don't check them as a matter of course.
Ice at $4 to $8 in the BVI is still cheaper than in the Grenadines where I've been quoted up to $16 for a bag (I passed).