This one of the reasons why you will see prices for drugs like Insulin going up with each manufacturer raising price in lock-step. The drug makers are not competing to sell the consumer insulin, they are competing to get on the formulary of the PBM.
And if you're a smart insurance company, you do your due diligence and don't negotiate a contract where you have no transparency into the PBMs costs.
It is clear you don't know much about PBMs. Maybe try to rephrase your comment as a question.
Express Scripts Holding Company is the largest pharmacy benefit management (PBM) organization in the United States,[20] with 2013 revenues of $104.62 billion.[21] In 2012 Express Scripts' $29.1 billion acquisition of rival Medco Health Solutions (once the nation's largest PBM) created "a powerhouse in managing prescription drug benefits."
The discounts are passed through via lower insurance premiums. You don't really see that since your employer pays the premiums.
And yes, ESI is a $100B per year company with $3.4B in net income.[1] A stunning 3.4%. Not that different than a grocery store.