I guess they’ve based it on the fact that if you were able to get 5x 30 day extensions off of a visa exempt entry (back then they were 30 days on entry) the cost of each extension would (almost) add up to the 10,000 baht fee.
I personally can’t see many applying when they can bounce out and back for another 180 days for much cheaper than that.
The YouTube video of the guy from the MFA spouting the rules of the DTV stirred up the pot. He claimed there would be no requirement for filing of 90 days and extensions would be 1,900 baht.