Medicare/Medicaid tend to pay less than private insurance, however lots of places accept it because that gives them access to a bunch of potential clients.

Leveraging your user base to get a discount from a provider is normal and expected.

The problem is when insurance companies demand a particular discount and providers given them that discount by raising their prices.

Certainly a 70% discount is a sign of a bad price (assuming it isn't part of a cost normalization scheme where some services get deep discounts and others are paid with little or no discount aka "I get 70% off dangerous surgeries but I will pay 110% of simple ones")

However if instead the normal price was $200 and they accepted $150 to get access to the network that is normal.