What documents and fees are required for extending a Non-B and Non-O visa in Chiang Mai?

I would wager neither of you have actual VISAS anymore and both of you are on 'extensions of stays' from the immigration office.

I'd go to the immigration office and get the hand out of what you will need. For sure you will need documentation from your employer to get another year extension based on employment, and I also believe you will need the employer to state it's okay that your spouse gets an extension piggy backed off yours (trailing spouse extension).

OR

Post on a Chiang Mai group and see what people up there say. Personally I'd go get the hand out and talk to the immigration office in person because I would want to pull it off in one visit rather than try to apply, find out you need this, get that, go try to apply, find out you need something else.

AND

you are correct, ALL extension of stays inside the country at the immigration office cost 1900baht no matter the length of time they're issued for