What are the financial requirements for extending a marriage visa in Thailand, specifically regarding income and savings?

There is no combination method allowed for extensions based on marriage (or raising children) you either bank 400K baht a month for 2 months before you apply for the year extension OR you have 40K baht in income every month for the previous 12 months before you apply for the year extension.

If you legally work here with a work permit on a salary of at least 40K baht a month you can use that. IF your consulate here doesn't issue the affidavit of income from abroad notary letter you need to transfer in to a thai bank account in your name only a minimum of 40K baht a month each month every month for the previous 12 months.