Yes, there are multiple flights from Bangkok to Thessaloniki for under ฿21,767. The cheapest flight booked recently is on KLM for just ฿20,523, but on average you can expect to pay ฿36,662.
There are no direct flights from Bangkok to Thessaloniki, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
You can imagine temperatures to be in the 36 °C range in April. The weather in Thessaloniki is generally colder than Bangkok's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.
For as low as ฿20,524, KLM can have you in Thessaloniki from Bangkok in 36h 15m. This flight may include one or multiple stops.
Cheapflights users have booked flights from Bangkok to Thessaloniki from ฿20,524 return. Another airline that can offer you cheap pricing is SWISS.