If doing SJU then look at Intercaribbeans website, book directly with them, go down and back off weekend and the fare is $167 under the fare saver...you could also get the cheapest Jet Blue fare and probably be closer to 400-450 all in DCA - SJU- EIS...

Caveat is again by booking separately your first leg carrier (jet Blue) isnt responsible for getting you there all the way