This is a failure of the free market. When companies get too big, and no longer compete, they should be regulated. The free market only works when there's a ... free ... market.
That's what I've heard, though I don't any data to show.
In general all medications are the most expensive in the US, often by a lot. For two main reasons:
1. All other countries are to some extent "free riding" on the US market, and refusing to pay full price.
2. The FDA is doesn't approve a lot of things. So many cheap foreign medications are illegal to import. I heard that many forms of insulin are in this group.