Where does the money come from in Medicare-D?

For example, suppose somebody gets a 3 month supply of various meds, and the
pharmacy claims that the retail price is, say, $180, but the premiums are
$20 per month,

Also assume that co-pays are $0.

So who pays the other $120(180-60)?

I'm thinking of a case in which the Explanation of Benefits states that the
pharmacy was re-imbursed the full $180 by the insurance company, i.e., the
medicare price was the same as the retail price..

What about Wal*Mart.  They have a list of generic Rxs that they sell to
people without insurance for $4 cash.  I had one that would have been $4 if
I had paid cash.  I have insurance with zero co-pay for generics.  But the
price listed for CMK Medicare was $34.46!

So do they get $30 more for filling it on Medicare as opposed to $4 if I had
them bill it as cash?
Or is it like most insurance payments?  That is the insurance company gets a
big discount and pays only a percentage of the billed price?