Nassau woulld likely be the least expensive flight option. I do find it rather dirty and boring, but I suppose there's enough to see & do the first time.

I used the Travelocity Dream Maps feature (sorry, no link but you can find it on their site map) to scan for what was low cost. Flights to Nassau were $90 RT from FLL (be sure to look using MIA, FLL & PBI as jumping off points). Other options under $300 included San Juan, Kingston, Providenciales, Grand Cayman, Marsh Harbour. Never been to PLS, so I think that would be my choice.