From memory.....costs may be a little off, but close. All costs are per person, one way: Ferry $60 Luggage $5/bag (including carry on roller bags) BVI departure tax - $20 Gratuity for bag guys $as you please Caribs in route - $5/ea Taxi from CA ferry terminal to STT - $10 (incl tip)
All said and done, I usually figure ~$100 pp each way.