How does the stay extension work for a 6-month Thai visa, and what are the fees involved?

Every time you enter you'll be stamped in for 180 days, each entry can be extended 180 days (1900 baht, it is assumed as this is the normal extension fee), or you can leave and return and be stamped in again 180 days. Nobody knows how the extension scenario will eventually play out it, it was suggested that paperwork/proof may need to be resubmitted. Right now it's recommended to bounce rather than extend.

not one single person has been ON a DTV entry stamp long enough to have applied for the one 180 day extension you can get per entry yet.

I mean face it, the visa itself is 9 weeks old (63 days 😉)

It's gonna be another 3-4 months before anyone has reports of what is required to get the in country extension and what the cost really is (there are reports it would be 1900baht AND reports it would be 10K baht)

Remember though the visa is a FIVE year multiple visa, meaning every time you exit/re-enter you get a new 180 day stamp. You may never NEED to get an in country extension if you just bounce out/back at the end of your entry stamp for a new one