It's 10,000 baht for the DTV extension. I guess I'm assuming everyone is like me -- I'd rather spend that money on a flight and take a holiday instead of giving it to immigration.
It's not yet established what will be the cost of DTV extension. Most think it will be same as other visas, i.e. B1,900. Also if you travel out and back in within the 180 days, the 5 years visa will cost B10,000 total (ok plus travel costs).