Peak travel to Thailand (dry season runs from Dec - April) with flights starting at $401 RT on China Southern Airlines. We don't typically share Chinese airlines, but this is the lowest price we've seen by far and especially during peak travel season.
If you book...don't miss the gorgeous Phi Phi Islands.
AIRLINE: China Southern
TRAVEL: Cheapest dates are in March/April/May. Flights in high 400s/low 500s starting in February and running through May. Sample dates: April 17-27 with a 17 hour layover in Guangzhou, China.
BOOKING: Search on Google Flights. Booking options from there included Priceline, Expedia, Orbitz, or booking direct with airline.