Agreeing a price for any drug is a tricky business.
In the UK, the National Health Service is the main buyer and prices are set through a voluntary scheme between manufacturers and the government, trying to strike the right balance of serving patients and generating money to keep the drug pipeline going. Profits are capped to stop prices creeping too high.
In the US, the buyers are private insurance companies as well as the government through the Medicare and Medicaid system. It's a market and prices can go up and down, depending on what people are willing to pay.
In recent years, pharmaceutical research and development has slowed and companies have to think carefully about what they invest in. Blockbusters such as Viagra pull in money, but orphan drugs for rare diseases can be less attractive. Not many patients use them, and so turning a profit may be difficult.