Fly from Baltimore to San Jose, Costa Rica on Delta Air Lines for $285 roundtrip. This fare is widely available from late August through early February of 2019, but books into the basic economy fare class. If you find that too restrictive, you can always bump up to "Main Cabin" for $15 each one-way bringing the total to $315 roundtrip. Still a solid deal.
Searching on the Delta website, I was able to find seats departing Baltimore BWI on Monday, December 3, returning from San Jose SJO on Thursday, December 13, 2018.
Need a hotel in San Jose? Here's what's available for the mentioned dates on Hotelwatchdog.