Prices to SXM from the USA are rather high. It costs considerably less to fly from the USA to Barbados, for example. I like SXM better than Barbados.
Why? Demand is one reason, I suppose, but---
American Airlines, which is the main US carrier in the Caribbean, tries to shake down the island governments for subsidies in return for serving their island. SXM probably doesn't ante up the cash.
I'd wager that landing fees are high at SXM. IMO, the Dutch side of SXM most likely has "unusual" financial situations - they cost money.*
A new/improved terminal is about to be built at SXM, at a cost of about US$100 million!
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There is a departure fee of US$20 for leaving SXM to all but the neighboring Dutch islands. Passengers have to pay this at a window in the terminal ... BUT I was pleased to discover that AA now includes this in the ticket price. No visit to "the window" is needed.
I just keep following the AA web site until they have a special - There's one right now for $500 from WAS right now (vs $400 to BGI) departing 1st April or later.
SC
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* As an example, I was told by a Dutch-side person that the proprietor of one of the Dutch-side hotels would be arrested for financial irregularities if he ventured to the French side.
The Dawn Beach Hotel (Dutch side) was heavily damaged by a hurricane a few years ago. It wasn't rebuilt. Reportedly the insurance agent had been pocketing the money - not actually buying insurance.