In a medical system notorious for opaque finances and inflated bills, nothing is more convoluted than hospital pricing, economists say. Hospital charges represent about a third of the $2.7 trillion annual United States health care bill, the biggest single segment, according to government statistics, and are the largest driver of medical inflation, a new study in The Journal of the American Medical Association found. A day spent as an inpatient at an American hospital costs on average more than $4,000, five times the charge in many other developed countries, according to the International Federation of Health Plans, a global network of health insurance industries. The most expensive hospitals charge more than $12,500 a day. And at many of them ... emergency rooms are profit centers. That is why one of the simplest and oldest medical procedures - closing a wound with a needle and thread - typically leads to bills of at least $1,500 and often much more. At Lenox Hill Hospital in New York City, Daniel Diaz, 29, a public relations executive, was billed $3,355.96 for five stitches on his finger after cutting himself while peeling an avocado. At a hospital in Jacksonville, Fla., Arch Roberts Jr., 56, a former government employee, was charged more than $2,000 for three stitches after being bitten by a dog. Insurers and patients negotiated lower prices, but those charges were a starting point. The main reason for high hospital costs in the United States, economists say, is fiscal, not medical: Hospitals are the most powerful players in a health care system that has little or no price regulation in the private market.