We are planning to travel to San Juan for a cruise in December. I have been constantly monitoring airfares between Portland and San Juan (albeit a little early yet to buy tickets). Since it's only a 2 1/2 hour drive, I've also been looking at fares out of Seattle. In general, JetBlue seems to have the best prices; right around $400rt (if you don't mind flying on a red-eye via JFK; depends on the days your flying as well; also, you can only book 7 months out on JetBlue). The flight leaving San Juan (there's only 1) leaves at 2:45, so that works out well if you want to leave the same day your cruise arrives back at port.
As Karen mentioned, prices do change and you're never guaranteed of getting the best price. Check all the usual suspects (Travelocity, Orbitz, Expedia). If you're adventurous, give Priceline a whirl. I'll let you know if I come across something else.