The annual flu shots that are free to those with health insurance are not immune from the convoluted and contemptible price-gouging that plague the US healthcare system.

Health insurance companies pay wildly different amounts for the same vaccines depending on how negotiations go with individual medical providers across the country. In some cases, providers have forced insurers to pay upward of three times the price they would pay to other providers, according to an investigation by Kaiser Health News.

The outlet noted that one Sacramento, California, doctors’ office got an insurer to pay $85 for a flu shot that it offered to uninsured patients for $25.

Though $85 might seem like a trifling amount in the bloated scheme of the US healthcare system, such prices quickly add up as tens of millions of people receive a flu shot each year. And while the Affordable Care Act requires insurers to cover the full costs of all federally recommended vaccines, including the flu vaccine, any extra costs to insurers get passed on to patients through higher insurance premiums, economists told KHN.

Looking further at what insurers paid for flu vaccines, KHN found that costs spanned the whole range from $25 to $85. A doctor in Long Beach, California, got insurer Cigna to pay $47.53 for a shot, while a CVS in downtown Washington, DC, got $32 from Cigna for the same shot. A CVS just 10 miles away in Maryland got $40.

My insurance did a little bit better than those. My doctor’s office in the District of Columbia initially charged my insurer, Aetna, $35 for my flu shot, and Aetna paid them the negotiated rate of $24.50.

But that’s still significantly above federally negotiated rates. The Centers for Disease Control and Prevention negotiated a price just under $14 for the same shot. The agency reported a private-sector cost of around $18. Likewise, the Centers for Medicare and Medicaid Services pays out $18 for the vaccine.

Aetna paid around 35% more than that for my identical shot—and there was no way for me to know that before I got the shot. Hidden negotiated rates make it impossible for patients to shop around. And this isn’t just a problem for flu shots. Wild price variances occur for everything from diagnostic scans to surgeries.

“We don’t have a functioning health care market because of all this lack of transparency and opportunities for price discrimination,” Glenn Melnick, a health economist at the University of Southern California, told KHN. “Prices are inconsistent and confusing for consumers,” he added. “The system is not working to provide efficient care, and the flu shot is one example of how these problems persist.”