Check out this thread on FT, and book fast if you are interested.
TACA earns 100% RDMs on UA and they offering roundtrip fares for as low as $134 r/t (including taxes) from New York, Washington, and Chicago. This is likely a "sale" fare.
If you are creative in your routing, you can visit Costa Rica, El Salvador, and Panama in the same trip.
This fare will likely be dead in a couple hours. Book now (I used Orbitz) and ask questions later–you will have 24 hours to cancel.