Is an income letter from my embassy valid instead of having 400k Bht in my bank account for a visa?

For a marriage visa/extension inside the country at the immigration office; if your embassy here issues the affidavit of income from abroad letter you can use it. (y) (in fact you HAVE to use it if you're using monthly income method :O)

If they don't;

you either bank the 400K baht

OR

you transfer in to the country 40K baht a month for the previous 12 months before you apply for your extension

OR

you work legally here and make a minimum of 40K baht a month salary.