Well, they know that many people will be going there for Christmas, most to visit relatives and so forth. I would wait a few more months, and be cognizant of the most expensive times to fly. Generally from Dec 15th-Jan 6 is the most expensive season. So if you fly on Dec 14, and return after the 6th, you'll probably save a lot of money.

Using NWA miles, I've found a lot of availability to SJO on CO or DL, although sometimes I end up getting standard FC seats as opposed to Y.