My 30 day extension on my 60 day TV has cost me 15000 baht, the cost of changing my original return ticket to cover my extension was 13000 baht.
I normally expect around 5000-6000 baht, but it appears I have picked a busy passenger flight that's the reason given.
Correct me if I am wrong but you can't buy a 90 day return on a 60 day TV, due to having to submit flight confirmation and days staying in Thailand on the visa application.