Overcharging on Treatment Plan - HMO

I have a proposed treatment plan for a crown for $1,275. I reviewed with Metlife HMA and they said I was only repsonsible for $500. The dentists is participating in the plan. I understand that they recover very little from HMO's but are they not contractually obligated per the terms of the plan as a participating provider? I have actually gone thru this with another dental office and was able to recover overcharges, after which they dropped out of Metlife. I am down to this one dentist who is participating in this area. Any help on approach would be greatly appreciated.