The stomach cramp and nausea began one hot Friday evening in August, midway through a vacation on Martha’s Vineyard. The next morning, nearly doubled over in pain, a patient who we’ll call "Nancy" walked gingerly into the emergency room at Martha’s Vineyard Hospital.

Nancy is a 55-year-old former nurse who would prefer not to use her real name because she works with the hospitals in this story.

Even Nancy, who spends hours every day focused on health care costs, would gasp when she saw the bill for this visit.

In the ER, a doctor poked at Nancy’s tender belly and took blood for tests and a urine sample. The doctor ordered a CT scan of Nancy’s abdomen and pelvis, using contrast. It showed bulges, inflammation and thickening in Nancy’s colon. The diagnosis: uncomplicated diverticulitis. Nancy filled a prescription for an antibiotic, took some Advil, and felt better after a few days on a clear liquid diet.

Five weeks later, the diverticulitis monster invaded Nancy’s intestines again. This time she went to an urgent care center closer to home, run by Beth Israel Deaconess Medical Center (BIDMC). A doctor there ordered the same single CT scan of the abdomen and pelvis, again with contrast.

Nancy says the care she received at both places was great. But a month later, when she received the bills and her insurance company's explanation of benefits for both visits on the same day, she was stunned.

The explanation of benefits show Blue Cross had paid Martha’s Vineyard Hospital almost seven times what it paid BIDMC’s urgent care center for the same CT scan — $3,888.76 vs. $574.97.