In STX where Seaborne is, my friends typically would pay 700-1000 a month for a place. You will need a car in St. Croix, so factor that in. In San Juan you can get around with out one, and the way they drive there, dont buy a nice one anyway. Rent in SJU is typically a little less that STT and STX, and a whole lot less if you move to the other side of the airport in Los Angeles, instead of Isla Verde.

The best financial deal in the carib though: fly for Cape Air, Live in Ponce: rent 350$ month all included, buy a 500$ Puerto Rican car and have some fun.