To clarify---you need to provide negative RT-PCR results to apply for entry to St. Kitts Nevis 72 hours prior to arrival.
You also need to take a RT-PCR test once you arrive ($150 per person). If you are at a hotel they will administer the test there. Once you receive negative results you may leave the hotel.So it is a roughly 24 hour quarantine in place.
You will also need to arrange another test for re-entry to US--also $150 per person I believe.
We figure between pre-departure testing, arrival tests then tests for US- re-entry---probably $1000 for the 2 of us. Not insignificant.