Testing is being done for both employees (now) and passengers (mid-November). The capacity is reported to be 400 tests per day.
The cost for the Anibody test is $75. You can also get all 3 tests (1-3 day, Rapid, and Antibody) for $215.
The testing center is in Terminal E, the international terminal, which isn't all that convenient for anyone traveling to SXM.