If you have your treatment plan in writing, which outlines the cost proposed, that would be your best weapon. If, after a treatment plan is proposed, the dentist did extra work or charged more without modifying the terms, your original contract stands. Even without a written agreement, the dentist has violated a verbal contract by pulling a bait and switch. Dentists MUST tell you in advance how much something will cost, especially if it is higher than the original quoted price.
If you did NOT sign an agreement stating that you knew what you were being charged and that you agreed to $20,000, that dentist will have a difficult time collecting. Most states have dental complaint boards. Please file a complaint immediately.
Call the dentist and do not back down. Let them know that you will be filing a complaint with your state and that you will not pay for treatment you did NOT authorize-Let me know what state you are in. I will find the website for you.
My best friend justt went through the same thing. She ended up paying the original price-nothing more. She fought it and did NOT back down!