We always charter from either STT or SJU to VG. Our group of four always needs the "bigger plane" due to our size and chronic overpacking so I have done this type of math. Going with a private water taxi from STT to VG always priced out comparably to flying so we fly.

It might be worthwhile to asking your rental agency for options.

If you need a private driver on STT, Christopher has treated us well since 2005 - https://www.christaxiandlimoservices.com/services I see there is an option for water taxi to the BVIs.