If you are married to a Thai and living in Thailand, and able to handle all the paperwork/procedures yourself (no lawyer costs) cost should be minimal.
If you are foreigners wishing to adopt a Thai child and take them out of Thailand, it can be complex and expensive. I know a couple who did it (actually the child was stateless).
The Department of Children and Youth in Bangkok oversees/approves adoption in Thailand.
The website https://www.dcy.go.th/ isn't exactly user friendly, but if you (maybe your wife, if you are not fluent in Thai) contact them and ask to speak to someone in the Child Adoption Centre, I'm sure you will get the best advice.