Had a similar situation recently - after hours on the phone I learned that insurances have a 'contracted rate' with in-newtork offices. These are pre-determined and they can't charge you more.

I would ask the dentristy for the 'code' assigned to that procedure (They have to have one in order to bill the insurance company). Then call your insurance and ask for the "contracted rate".

In my case, the office over charged me and ended up refunding me the difference.