Standardized pricing gets rid of the problem of charging different customers (slightly) different pricing, and having massive negotiations on a per-carrier basis. This sort of thing isn't a matter of "competition drives down cost". It's just stupid and inefficient.
Also, Medicare and Medicaid can't negotiate drug prices.
Medicaid varies because states have way more control and many states (especially Republican-leaning ones) have slashed their rates as part of a concerted effort to kill the program. Not that it matters - in Texas you generally can't qualify for Medicaid if you're childless or have a job. The income requirements are insanely low.
If a doctor doesn't like the rates Medic[are|aid] are paying, they're free to not accept Medic[are|aid] patients.
There are options for transparency that don't involve a government takeover of health care. But the brinksmanship in the politics of health care --- almost entirely the fault of the GOP --- is keeping us from exploring solutions.