@David *******
these are guesses, but they're not without basis. There are other visas which cost 1,900 baht to renew. When you renew them, you're required to show you've maintained most of the requirements for initially getting the visa.
To extend an ED visa, you must still be taking the class. To extend a non-O, you must still meet the financial requirements.
It makes sense the DTV would be the same.
But this is Thailand. We'll see.