Human insulin remains one of the few medications, along with thyroid hormone, where we can use the exact same molecule that is naturally produced in our bodies. Costs for human insulin should be, and often are, quite low, and there is no patent-based exclusivity. However the US market is dominated by analog insulins, which are much more expensive, and whose prices have risen steadily, as different insulin producers have raised their prices in near synchrony with each other (see Bloomberg news article link). The clinical advantages of analog insulins in people with type 2 diabetes are actually quite small, and are present for only a minority of those needing insulin treatment. Medicare/Medicaid spent $5.7 billion dollars on a single analog insulin in 2015, the second largest expenditure of any covered drug. Profits are important, but is affordable health care, and few medications are as essential as insulin.
See graph in this Bloomberg News article of lock-step price increases.
See Lancet article on costs of insulin.