As Glenn indicates the difference between a US boat and a BVI boat is $10/day/person. The ferry (assuming you're landing STT) is $65/person round trip. So if you make the assumption all else is the same there is a break even point of 6 days. But of course not all else is the same. Honestly... if you're budget is tight enough that $10/pp/day is a factor on where you want to point the boat I'm wondering if this might be the best vacation for you.