What are the costs and process duration for obtaining a retirement visa in Thailand?

Assuming you don’t already have a Non O visa you’d enter on a visa exempt or tourist visa. With at least 21 days left on that stamp you’d apply for a Non O in country provided you have managed to open a bank account and transferred 800k from overseas (unless your embassy issues income affidavits). Cost is 2,000 baht.

Once on that and it’s winding down you apply for a year extension with the 800k being in the bank for at least 2 months. You’d need to keep it in after for 3 months and then the balance not go below 400k for the rest of the year. Cost is 1,900 baht.

To get the next extension it would need to be back to 800k 2 months before applying.