Which place has cheap ticket for kickboxing (muay) Thai in Bangkok?

Lumpini and Ratchadamnoern are the biggest stadiums with the best fighters in Thailand, so they will be the most expensive. I would suggest trying one of the other stadiums if you want to watch fights cheaper. But Westerners will always pay more, usually at least 1000Bht to get into fights. It'd probably be worth paying a bit more to see the champions fighting in Lumpini.