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.