The fine depends on the number of days you have overstayed. The minimum charge is 500 Baht per day. This goes to a maximum of 20,000 Baht for overstays of 40 days or longer.
The good news is that if your overstay is under 90 days and you pay the fine then leave the country then you won’t have any problem returning to Thailand at a future date. There won’t be any black mark next to your name.