Ukrainan flag carrier Ukraine Airlines brings some interesting fares when booking an open-jaw flights from Europe to Thailand. The cheapest flights to Bangkok are currently available for Ł238 (roughly €292). In this case you have to look for departures ex Athens and you have to return from Thailand back to various European airports. Take this opportunity and visit not just Bangkok but also historical Athens during your trip to South-East Asia!
Since Athens are not that easily reachable by low-cost airlines this deal is mostly targeted to travellers that wants to visit more interesting places during their holidays. We recommend to check your connection possibilities via Skyscanner.
To reach the cheapest fare possible you have to look for returning back to Germany, Brussels or London. You may however consider to return to many other Euroean cities such as Paris, Milan or Vienna. Current promotional deal of Ukraine International offers to book those flights till the end of June 2014. (based on your outbound flight to Asia).
If you are considering to fly to Bangkok (even during main tourist season) you may consider still active promotion of China Southern Airlines. They are offering cheap open-jaw flights to Thailand when departing from Amsterdam and returning to London for Ł333!
Booking sample of a cheap open jaw flight to Thailand from Europe for just Ł238 below. In this case you would be departing from Athens to Bangkok on 4th of June and returning from Thailand back to Frankfurt on 16th of June 2014.