Gas prices on the Dutch side are fixed and the latest price I see reported in the Daily Hearld is 1.999 NAF per liter. With the 1.75 to 1 exchange rate, the Dutch side price per liter is 1.142 USD.

The Cadisco on the way to Orient Beach and the one on the Marigot side of the French bridge offer 1:1 with cash (Euro to USD). So if their prices are less than 1.142 euros, which I suspect they are, then those are probably the cheapest on the island.