Dentist gave estimate on cost of nothing but I end up with $511 bill; is this legal?

My wife and I recently had dental work done at our dentist office. We asked prior to doing so how much it would cost and they said nothing with the insurance we have. Two weeks later we have a $511.00 bill. We called the dentist office and they said that what they gave us was an estimate and it does state that on the actual written estimate, but doesn't it need to be within reason such as 10-15% of what they state. This seems like fraud to me. BTW we live in the state of Washington