I recently got a 30 day extension to a 60 day tourist visa for 4,000 baht via an agent. (To avoid going to Jomtien immigration and queuing for hours to get covid.) She told me I could have 60 days extension for 6,000 baht. So that would make 4 months from an online tourist visa.
She also told me it would be 25,000 for a retirement visa, even with the 800,000 in the bank.
A problem not mentioned with the agent but no money in bank method is that if you annoy someone important, like the police trying to rip you off, all they have to do is check your bank records and file criminal charges.