What are the financial requirements for obtaining a marriage visa in Thailand?

400K baht in a thai bank account in your name only

Most immigration offices will not accept the 40K baht "monthly income" method for proof of funds on a marriage visa/extension unless you legally work inside the country with a work permit

Check with the office you're going to apply for the visa/extension at and see if they will allow monthly income method by international transfer (which would be 40k baht a month, each month, every month for the previous 12 months be transfered from abroad into a thai bank account in your name only before you apply for the year extension)