My son needed dental work from our regular dentist(been with for about a year and a half), they had told they wanted to do several things that would cost over $1000 after my insurance. I told them to wait till January when insurance would refill. They called me back and said they could fix worse tooth with a crown and cavity next to it for $120 out of pocket. I accepted this. On my sons appointment to put in permanent crown they said they had miscalculated and I owed them $925 more. I know it's unethical, but is this legal? By the way there is no way in hell I will pay it.
You have to look at the contract, which is that thing they made you sign before the treatment. The primary rule is that the written contract rules (look for a clause saying that "this is the whole agreement" – there is a legal principle, the 4 corners rule, that precludes verbal statements from determining what the agreement is), and the contract probably has a specific statement about your financial responsibility. They may be contractually limited to certain amounts if you are an "in network" patient (because of their contract with the insurance company), but there is always language saying that if the insurance company doesn't pay (not in-network, or some deductible or other coverage-related issue), their friendly cost estimates are not price commitments.
You can refuse to pay and they can take you to court; you can make your argument that they promised to do the work for $120. They will counter-argue that they gave you an estimate of the actual cost based on information that they had available, but you agreed to pay the full amount, insurance-based estimates notwithstanding. You can argue that at no time did they ever tell you what the worst-case scenario price would be, and they can argue that a reasonable person would know that $120 is not the full price for a crown and filling. They can argue on grounds of unjust enrichment that you owe them more money. It is not uncommon for people to misinterpret dental estimates as concrete prices.
You do not owe the money
Your contract with the dentist was clear: $120 + insurance for a crown and filling.
If this were a construction contractor or a car salesman, the question wouldn’t arise: if they made a mistake in their pricing, that’s their problem. It’s no different for your dentist, they are bound by what they agreed in the contract.