Kathy Meinhardt understands business. She runs a small graphic arts business out of her home in Northern California.

But what she can't understand is what happened after she had to have a second round of blood tests - nearly the same tests she'd had just months before.

Click: HealthCareBlueBook helps people find fair health care prices.

This time she went to Queen of the Valley Medical Center's outpatient lab, in Napa, California, just down the block from Quest Diagnostics, the independent lab where she'd been before.

"I got a big surprise after the tests were done. It was a bill that blew my mind," Meinhardt said. "It was outrageous."

Meinhardt's total bill for lab tests at Queen of the Valley Medical Center was more than $4,000 and even after her insurance adjustments she was still on the hook for more than $3,000.

Related: Sticker shock: Investigating the high costs of hospital bills

As part of ABC News' ongoing investigation into hospital bills, ABC News did the math for 10 blood tests Meinhardt had at both places just four months apart.

For those same basic tests, Queen of the Valley Medical Center charged 2.5 times what Quest charged.

But because Meinhardt's insurance company does not have the same negotiated rates with providers, she had drastically different out-of-pocket costs. Meinhardt's out-of-pocket cost from the Medical Center outpatient lab was $1,850, seven times as much as Quest Diagnostics, where she was charged $260.

ABC News

It was Aetna, her insurance company, that suggested the Queen of the Valley Medical Center when Meinhardt inquired. Both facilities are "in-network" based on her policy.

But according to HealthCareBlueBook, a website where people can go to find fair health care prices, the cost of in-network procedures can vary by more than 400 percent.

Related: "Real Money" shows you how to save on emergency room bills.

ABC News asked Queen of the Valley Medical Center for an on-camera interview and was told its CEO, Walt Mickens, was unavailable.

In an email statement to ABC News, Mickens said: "We are always concerned with the issue of rising health care costs. But comparing a hospital with a free-standing facility is an apples-to-oranges comparison. Unlike free-standing facilities with fixed hours and a limited, narrow scope of services, hospitals provide life-saving services such as, in our case, trauma care, and are staffed 24 hours, seven days a week. In addition, hospitals care for everyone, regardless of their ability to pay."

Read CEO Walt Mickens full statement here.

Meinhardt told ABC News she didn't realize things would be different because she went to the Medical Center's "Outpatient Laboratory Services" facility, open Monday through Friday from 7 a.m. to 5 p.m., according to its website.

Queen of the Valley Medical Center also referred ABC News to the California Hospital Association in Sacramento, Calif.

Jan Emerson-Shea, vice president of external affairs at the California Hospital Association, told ABC News that "each individual hospital sets its own pricing formula."

Hospitals typically base their charges on something called a "chargemaster" - an internal document usually filled with tens of thousands of codes and charges for services and products.

In almost all states hospital chargemaster rates are privileged information nearly impossible for everyday patients to see.

Related: "Real Money" ways to save families on hospital costs.

But in May 2013 Medicare released actual charges Medicare was billed from hospitals across the country revealing wide variation in charges for 100 of the most common inpatient services and 30 common outpatient services.

California, however, is one of the only states that requires hospitals to make their charges public on a government website.

The 2013 Queen of the Valley Medical Center chargemaster has more than 8,000 different codes and charges. It lists separate fees depending on whether a procedure is performed as an outpatient or inpatient. But the information isn't always helpful for patients who often can't interpret billing codes and descriptions, are unaware of what procedures or tests they may need, or are unsure what financial arrangements their insurance company may have with the hospital.

Aetna told ABC News: "Overall, we have a good contract with Queen of the Valley but that contract, as with any of our hospital contracts, establishes a discounted payment rate for a given service, but it does not establish the hospital's charges for that service."

Another obstacle to price transparency is what health policy experts refer to as "gag clauses," actual language in contracts between health plans and health care providers that specifically forbids the sharing of pricing information with consumers.

In fact, ABC News was not able to obtain exact negotiated rates for several lab tests Meinhardt had. In an email, Quest told ABC News "negotiated rates with health plans are confidential," and while Aetna provided ABC News with average lab test prices for several nearby California counties, Aetna told ABC News "information on rates Aetna negotiates with all providers is considered confidential."

ABC News obtained pages from two separate contracts between providers and hospitals, both not involved in Meinhardt's case, which contain "gag-clauses."

One of the current hospital-provider contracts obtained by ABC News in part states that "fees for covered and other health services," among other things, must be kept "confidential" and may not "…be communicated to the public…"

Francois de Brantes, executive director of the Health Care Incentive Improvement Institute, said: "It's impossible to create a functional market for health care services if consumers aren't allowed to know the price of those services. That's absurd and should be banned."

And because so many hospitals, doctors, labs and insurance companies have negotiated rates that are confidential, the true cost to a patient is not something most hospitals will share in advance.

"It's an abuse of the system, and it's financially harming millions of Americans every day," de Brantes said.

The hospital industry also cites rising overhead and blames confusing bills and high costs on "cost shifting." It says people like Meinhardt pay more to cover those who pay nothing and to makeup for what hospitals say are inadequate payments from Medicare and Medicaid.

"It's complex, hospital finance discussions, and I wouldn't expect any lay person to ever understand any of this," Emerson-Shea said.

Aetna has since worked with Meinhardt to renegotiate the Queen of the Valley bill down to $900.

Aetna told ABC News it was currently "in the middle of contract negotiations" with Queen of the Valley Medical Center's owner and "cannot comment at this time about what the structure of the contract will be."

Meinhardt told ABC News that Aetna's negotiated rates at Queen of the Valley "are so much higher than the individual lab and this is something that they do not tell their subscribers."

However in an email, Aetna told ABC News that Aetna members can log in to the Aetna website to "estimate the out-of-pocket costs for services by a doctor or facility."

Meinhardt's advice: "Play detective, you have to play accountant … [and] give yourself the time to get those answers."

ABC News' Dr. Mark Abdelmalek, Rebecca Jarvis and Julia Bain contributed to this article.