Yes, there are multiple flights from Thailand to Chittagong for under ฿ 30,019. The cheapest flight booked recently is on Multiple Airlines for just ฿ 29,839, but on average you can expect to pay ฿ 33,051.
Biman Bangladesh is one of the carriers that may have flexible change policies. You can use our site to filter for other airlines that may also have this kind of flexibility.
There are no direct flights from Thailand to Chittagong, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
Departing from Thailand to Chittagong will take you 16h 55m.
Flying on Emirates is currently your cheapest option to Chittagong, with prices starting at ฿ 30,440.