We were in St. Thomas last year, and thought that we would get a better deal if we just walked up to the cable cars and paid there. We found that it is the same price as purchasing from the cruise line.
I actually paid $17 on the ship for the tickets which were $18 at the gate. We went up pretty early and walked around the trails etc. Then we sat on patio at the bar and relaxed for an hour. It was well worth it and we got great pictures.