We're flying Memphis-Atlanta-San Juan-EIS next week for about $320 o/w. I later saw the same flights for $50 less. We've got roughly the same fare (going through Charlotte and maybe Miami) in January, February and March. We like to buy one-way tickets, so we're not locked into a return date.

Adding in taxi, ferry, and taxi to Trellis Bay, we wouldn't save going to STT.

You might find a good last-minute fare, but generally you will do better buying ahead of time. For example, AA had some good fares several weeks ago. Avoid traveling on the weekend, particularly around school holidays. Christmas and New Years are always very expensive.