I know that in St. Thomas things will be expensive. Crown Bay Marina came in at $4.15 per foot per day, with our a slight discount for monthly rates. Yacht Haven Grande is higher and I'm not sure about American Yacht Harbor in Red Hook.
With the ease of ferry access, it might be worth it.to consider Nanny Cay ($1.85/ft) or even Virgin Gorda Yacht Harbor ($1.30/ft) in the BVIs.
Only caveat there is that you'll need to import your boat ($202) if you plan on staying more than 30 cumulative days in the BVIs in a year.
SV Adagio, SM #209
St. Thomas (at anchor :)