From the way I read the chart, it would each $28 each way ($7 per person), if it is not late at night (there is a premium after certain hours).

Taxi rates: http://taxistmaarten.com/wp-content/uploads/SXM_Zones_and_Rates_Sheet_2014.pdf

If you are going during the day and want to take the bus and feel like a local, it is $2 per person each way.

While many say a car is a must, we don't rent one (and we usually stay 3 or 4 weeks) since when we venture away from Orient Bay, we usually enjoy alcoholic beverages. We don't want to drive after consuming the wobbly pops. Aside from safety, remember that the level of blood alcohol allowable in France is lower than in most places in the US. So for us, a taxi or a bus is well worth the investment.