Washington, DC - Costa Rica $307 RT on United Airlines via Priceline
Travel Availability is very limited to end of Oct-early November, then again Jan 29, 30 and Feb 5, 2017 ($312 on Feb 21)
Check date availability on Google Flights and use those dates to book on Priceline , free cancellations within 24 hours of booking
Please note that while this fare is valid at time of posting, fares can fluctuate in a matter of minutes/hours/days.
My sample travel dates: Oct 25-Nov 8 and Feb 5-14, 2017