Yes, there are multiple flights from Thailand to Yamaguchi for under ฿ 20,965. The cheapest flight booked recently is on ANA for just ฿ 19,108, but on average you can expect to pay ฿ 19,617.
There are no direct flights from Thailand to Yamaguchi, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
For as low as ฿ 20,127, ANA can have you in Yamaguchi from Thailand in 14h 50m.
Flying on ANA is currently your cheapest option to Yamaguchi, with prices starting at ฿ 19,108.