Yes, there are multiple flights from Bangkok Intl to Yamaguchi for under ฿ 20,975. The cheapest flight booked recently is on ANA for just ฿ 19,028, but on average you can expect to pay ฿ 49,082.
There are no direct flights from Bangkok Intl to Yamaguchi, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
Departing from Bangkok Intl to Yamaguchi will take you 12h 46m.
Flying on ANA is currently your cheapest option to Yamaguchi, with prices starting at ฿ 19,028.