Delta has a fare sale from across the USA from American Airlines hubs to several airports in Central America. The Flight Deal has found many routes under $300. Los Angeles and San Francisco are not currently participating airports.
The Flight Deal posted these routes early this morning and the fare deal has been spreading around the blogosphere, so there is likely less availability now.
I was not seeing any deals to LIR airport Liberia, Costa Rica from Philadelphia in several searches before I found a $274 fare in October. Many of the dates showing under $300 for PHL-LIR on Google Flights priced in the high $500s on Delta.com when I checked fares.
There are still plenty of routes and dates with under $300 round trip fares to Central America from each of the U.S. airports.
St. Louis STL
Central America Airports
Belize City, Belize BZE
San Jose, Costa Rica
Liberia, Costa Rica
San Salvador, El Salvador
San Pedro Sula, Honduras SAP
Tegucigalpa, Honduras TGU
Managua, Nicaragua MGA
Sample Delta Fares to Central America
Here are sample Delta fares I found at 12:30pm California time. Early June, September and October appear to have most lowest fare dates under $300 round trip.
Dallas DFW – SAP San Pedro Sula, Honduras $296 October 6-10.
Dallas DFW – MGA Managua, Nicaragua $300 October 6-10.
Phoenix PHX – MGA Managua, Nicaragua $290 September 13-17.
Phoenix PHX – SAL San Salvador $277 September 13-17.
Phoenix PHX – SJO San Jose Costa Rica $264 September 13-17.
St. Louis STL – SJO San Jose Costa Rica $269 September 10-14.
St. Louis STL – BZE Belize City $264 September 10-14.
Philadelphia PHL – LIR Liberia Costa Rica $274 September 30-October 4.
Delta Airlines Philadelphia – Liberia, Costa Rica LIR
$273.42 October 3-7, 2015
You will get more miles crediting flights to Alaska Airlines for 50% flight miles rather than Delta Skymiles 5x base fare.