We just left St Thomas about 36 hours ago. We didn't go over to Magens Bay this trip but it seemed like half the ship did. There were seven large ships in port that day. That was quite a few as they put ashore about 25,000 guests. Folks on the adjacent balcony did the Bay.
Besides two or three ship's shore excursions which included Magens Bay, there were hundreds of cabs, etc. available. It takes about 30 minutes one way by cab to get there. Ship excursions are about three times as much although you get to do other stuff like sightseeing besides time on the beach. You didn't say if you were porting in Havensight (WICO) or Crown Bay. About $8.00 pp from WICO or $12.00 pp from Crown Bay, one way.