Extension for other visas is 1,900 baht. No one has ever extended a DTV and so no one can give any definitive answer on that. However, we can safely say that the 5 year multi-entry DTV visa is 10,000 baht. It mentions nothing about a 10,000 baht extension.
you have 0 evidence to back this up so don't say it wont happen if you get the visa since you have 0 idea if that is true or not. Remember, immigration is a separate entity and they can do whatever they want and ask for proof of enrollment in your school / program when you try to re-enter. The fact is no one knows the answer to his question.
I'm in the same boat. Shoot me a message if you'd like. From what I understand you need to cancel your Ed visa and leave Thailand before it expires. Then apply for DTV outside Thailand and come back.