Our day trip on GB Express in Feb - by the airport was $90 + $13 Anguilla depart tax - round trip

The public ferry leaves from Marigot, which means a cab ride from the airport. You'll pay a french depart tax - $5, $15? each way and the $13 Anguilla depart tax.
So less money than a ride from the airport, but you have to add in the taxi cost.

Timing maight be an issue if you rushing to make a late ferry from Marigot.

J&B