CHICAGO — What do hospitals charge to remove an appendix? The startling answer is that it could be the same as the price of a refrigerator — or a house.

It’s a common, straightforward operation, so you might expect charges to be similar no matter where the surgery takes place. Yet a California study found huge disparities in patients’ bills — $1,500 to $180,000, with an average of $33,000.

“There’s no method to the madness,” said lead author Dr. Renee Hsia, an emergency room physician and researcher at the University of California, San Francisco. “There’s no system at all to determine what is a rational price for this condition or this procedure.”

The disparities are partly explained by differences among patients and where they were treated. For example, some had more costly procedures, including multiple imaging scans, or longer hospital stays. A very small number were treated without surgery, though most had appendectomies. Some were sicker and needed more intensive care.

But the researchers could find no explanation for about a third of the cost differences.

The researchers examined 2009 data that hospitals were required to submit to the state on 19,368 patients — 18 to 59 years old — with appendicitis. To get the fairest comparisons, the researchers included only uncomplicated cases with hospital stays of less than four days.

The study looked at what patients were billed, before contributions from their health insurance. The figures don’t reflect what hospitals were actually paid. Insurance companies often negotiate to pay less than what they are billed, and what patients pay depends on their health plans. Those least able to pay — the uninsured — could be socked with the full bill.

Uninsured and Medicaid patients had slightly higher bills than those with private insurance. Charges were highest at for-profit hospitals, followed by nonprofits. County hospitals, typically safety-net hospitals, had the lowest charges.

The costliest bill, totaling $182,955, involved a woman who also had cancer. She was treated at a hospital in California’s Silicon Valley. Her bill didn’t show any cancer-related treatment.

The smallest bill, $1,529, involved a patient who had her appendix removed in rural Northern California. Otherwise, the cases were similar: both patients were hospitalized for one day, had minimally invasive surgery, and had similar numbers of procedures and tests on their bills.

The lowest and highest bills were not freak occurrences; many cases involved charges well over $100,000 and under $2,000, Hsia said. Also, within geographic regions, the lowest and highest charges differed by tens of thousands of dollars.

The analysis echoes other reports, including a study of 66 hospitals in the U.S. and Canada that found charges for the same services varied widely in both countries.

Data from the federal Agency of Healthcare Research and Quality and the International Federation of Health Plans suggest the nationwide average price for an appendectomy is almost $28,000.