Dear jdemma,
Whether your provider can charge you a higher price that his contracted (negotiated) rate depends on the terms and conditions of his contract with your insurer.
The only way you can find out about this particular term is to contact your insurer and ask them that question directly. This will get you the information you need & inform them at the same time if their contracted provider in breaching his contract.
Sincerely,
Amir Mostafaie
Were there any extra things involved? Like anaesthetic? Always remember to read the terms and conditions carefully. What you have been told might not be the same as what the signed contract says.