You don't pay a departure tax, but there are fees - I think just when clearing back in - I forget. Always seems to be a different price, but it's fairly insignificant.

The clearing of customs is not a big deal. We take the opportunity when clearing in at St. John to stay a bit in Cruz bay - walk around, have a drink/lunch..etc.

Clearing in and out of the BVI, its all about timing - just don't do it when there's a ferry or a ton of other boats coming and going