The Westin at Dawn Beach is about as far away as you can get from Marigot. It's not a big island, and while there is a lot less livestock in the road than there used to be, there is a lot of traffic, people stopping in the road to have conversations, drawbridges that go up and stay up for awhile, etc. For you to get to Marigot going through the French side past Orient and through Grand Case would take over half an hour even if traffic is light.

Sorry I don't know too much about taxi fares. They are fixed rates, and not determined by meters, so make sure you and your driver are in agreement before you start out. SXM is one of the most competitive markets anywhere for car rental. If you rented a car 24 hours before you wanted to arrive at the ferry, and arranged to return it there, that would no doubt represent a good value, compared to taxi fares. The last time I took a taxi in SXM was 2+ years ago, Marigot ferry to SXM, and that was $20 + tip, with a stop at my favorite bakery. I'm sure it's gone up since then; SXM to Westin is a shorter distance; Westin to Marigot longer.

Enjoy SXM!