To extend in Thailand, every other visa I know about requires you to meet most of the same requirements as when you got it initially. If that's the case with the DTV, you'll need to show 500k in a bank, proof that you're still in a Muay Thai course or whatever basis you got in on. But we don't know yet.
As someone else said, you can probably just hop out of the country and come back in instead of extending in country.
Extensions for other visas are usually 1900 baht. We don't know that yet about the DTV either.
It's the airlines that have to foot the bill if Thai immigration turns you back. So they may require an onward flight. But I don't think they will for a DTV, unless they misunderstand it.
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.
There's no income requirement for the DTV. But yes, your business must be registered and you should have a separate bank account.
Most places don't accept retirement or investment accounts to meet the 500k baht requirement. So put that money in a savings or checking account unless you already have 500k in checking or savings. How long it needs to be in there varies by where you apply. Hopefully, someone with more specific knowledge of Chicago can chime in.