Full list of fines under the Land Traffic Act 1979 here:
http://www.ratchakitcha.soc.go.th/DATA/PDF/2563/E/195/T_0009.PDF
This was published about 2 years ago. It's all in Thai (which I can't read), but there's nothing over 1,000 baht listed.
Of course, the offence may come within some different legislation with higher penalties - but 20,000 baht, for overloading alone, seems improbable.