Here's the quote I got from Island Birds recently. These rates are from Isla Grande (taxi ride from SJU) to EIS. Rates from SJU are more, about +$1,000, if I remember correctly.

The roundtrip rates are:
$3350 for the 5 Passenger Piper Aztec
$4750 for the 9 Passenger Piper Chieftain

The rates do not include the BVI Arrival and Departure Taxes of $10 and $50 per person respectively.