What documents are needed and what is the cost for a 60-day COVID extension in Thailand?

Bear in mind that the so called "Covid extension" is very much in the hands of the immigration officer you speak to on the day. You may or may not get, it's unlike normal extensions which are pretty much "meet the rules get extension"

You’ll need this stuff. Not all offices have the same requirements so it may be slightly different depending on which of the 70+ offices you use.

A copy of your passport data page (signed),

A passport photo,

Proof of filing a TM30,

A copy of your departure card (TM6) (signed)

A copy of your latest entry stamp (signed)

A copy of any previous Covid extension stamps (signed) (Phuket requires this other offices may not)

In Phuket they require the copies to be in portrait not landscape. This means the top of your passport aligns along the shorter side of the A4. Except for the Data Page where most passports are “sideways”

(Other offices may not be as pedantic)

A filled out TM7 (application form for extensions) - the form doesn’t have a space for your email or phone number but just write it on the back (page 2).

A filled out STM2 (acknowledgement of terms of temporary stay)

A filled out “I don’t want to leave because of Covid form” - I have no idea if it has an official name or number.

And 1,900 baht.

The forms are available at your local office. Most have places on site or nearby that can do the copies and photos.

Many offices will give you a “under consideration” stamp requiring you to return somewhere from 5-14 days later where they will stamp for the remaining 60 days. If they do you MUST use the same office to “close out” that stamp. For example you cannot get a “under consideration” stamp from Bangkok and close that out in Chiang Mai 15 days later.

If it’s your first time applying for the extension they’ll date it from the day you apply, so you’d lose any remaining days you have on your current stamp. Subsequent applications would possibly be added on to the existing stamp.