Most small businesses (which are registered as a business with the MEA) use MEA type 2.1 Small General Service on Normal Tariff (Below 12kV).
For February 2023, the cost was 6.33 baht per unit. Here is an example where 3,684 units were used and the total cost was 23,330.40 baht.
https://www.mea.or.th/en/aboutelectric/116/280/form/21