Patent expirations don't factor into this article very much, but there's this:

The simple explanation for excessive drug prices is monopoly pricing. Through patent protection and FDA marketing exclusivity, the U.S. government grants pharmaceutical companies a monopoly on brand-name drugs. But monopolies are a recipe for excessive prices. A company will raise prices until its profits start to drop.

That monopoly is very fleeting.

By the time a drug makes it to market, it's typically been about ten years since the patent was issued. That gives a drug company about 8 or so years of exclusivity before any other drug maker can produce the drug without infringement.