The DTV doesn't expire when you extend your stay at immigration. You just cannot extend the same entry more than once, which means you'll have to leave Thailand when your once-extended time is up. But you can reenter using the same visa, as long as it's within its five years validity period.
Not sure about the cost of an extension. Initially, that 10,000 baht fee was indeed communicated, but more recently, it looks as if the standard fee for any extension (1,900 baht) will be charged. We might need to wait for the first actual reports to know for sure.