After David Hubbard underwent a routine echocardiogram at his cardiologist's office last year, he was surprised to learn that the heart scan cost his insurer $1,605. That was more than four times the $373 it paid when the 61-year-old optometrist from Reno, Nev., had the same procedure at the same office just six months earlier.

"Nothing had changed, it was the same equipment, the same room," said Dr. Hubbard, who has a high-deductible health plan and had to pay about $1,000 of the larger bill out of his own pocket. "I was...