@Antonia ********
, you would first apply for a 90 day Non-O visa when you have at least 15 days left on your current "admitted until" stamp. That costs 2000baht, and then you go back in two weeks and get it and a new 90 day stamp inked into your passport.
Then you'd wait until you have 30 days left on that new 90 day stamp and apply for a yearly extension of stay for 1900baht and once you got that (they're issued the day you apply) then you'd apply for a multiple re-entry permit for 3800baht.
So all in the hard costs at the immigration office are 7700baht
Of course it goes without saying you need to meet the financial requirements which are 800K baht in a thai bank account in your name for 60 days before you apply, or a letter from your consulate stating you get 65K baht a month in income or a combination of banked money and the consulate letter that totals 800K baht for the year.