Earlier quoted context omitted.
I don't know about your pharmacy but at my (major chain) pharmacy, if a drug price is less than the co-pay, I just pay the price. (Which is I assume the same situation as with Walmart.)
Without an external mandate to keep the price low, this should only happen rarely, because the chain will realize that they're underpricing the drug. If your copay is $10 and they sell it retail for $5, they'll make some kind of differentiation in the labeling to effect price segmentation and maximize the profit from each group of potential shoppers. That way, they'll be able to bill your insurance co $30, you'll pay…
All I can say is that's not my experience. At the large chain pharmacy I use, all of my regular generic prescriptions are priced under my modest co-pay. Prescription medications may not be the most competitive market but enough people are aware of price for ongoing medications that they will switch if a given pharmacy is consistently high.