What are the financial requirements for a retirement visa in Thailand versus a marriage-based visa?

The best part of the Non Immigrant O visa or the Extension of Stay based on married with a Thai National is that you are married with a Thai National, as long as she is your fiance you can not use this type of visa as a fiance meens not your wedded partner in Thailand. No idea if this is different in your country.

It looks like you're American. You should know that the US embassy does not verify income so the only way for you to use the monthly income method is to transfer 40,000 baht every single month into your Thai account for a year and then when you go for a marriage extension you would show proof of those transfers from the bank as evidence of your income. Then you would have to keep doing that forever as long as you want to stay in Thailand.

The embassies of some other countries will verify their income so they don't have to do this, but the US embassy stopped a while ago.

So if you hear people mention getting your income verified, know that it doesn't apply in your case.

you got your wires crossed a bit..

That you're going to build a house or that your thai wife has property doesn't cut any ice.

The financial requirements for a Non-O visa/extension based on marriage to a thai are you either bank 400K baht in a thai bank account in your name only for 2 months before you apply for your year extension, OR you transfer IN to the country a minimum of 40K baht a month each month every month for the previous 12 months <- meaning you can't use this method for the FIRST yearly extension (because you won't have 12 months of transfers)

The 65K baht a month you're talking about is for a retirement visa/extension NOT one based on marriage to a thai