Yes, it's $6/gal USD. They go by liters, and when converted, it works out to around $6/gal. I'd say at least $60-80 to fill it up if you run it completely dry. I don't think you'll use that much gas in St. Maarten. Maybe 5 gallons?
I wouldn't worry a lot about the price of gas. On an island that is only 34 square miles you most likely aren't going to use but one tank of gas. You can pay $50 a piece for a three hour shore excursion or pay for a tank of gas.
Yes, we complain about gas prices. But keep in mind most countries that have high gas prices also have social health and education (including college) paid for. If we want social health and education I guess we'll be paying for it in higher gas prices.