The bill shows they asked for $X, but the doctor already has a negotiated agreement with the insurance company that says they will accept $Y for the service.

Additionally, if you inform the office you are paying the bill yourself (without insurance) they usually give you a discounted price somewhere between $X and $Y.

In other words, almost nobody is paying the list price of $X. It's not really a meaningful number.

(And, as others have pointed out, the weak US dental insurance market means that actually dentists are pretty up-front about pricing in my experience.)