For bringing easy comparison-shopping to choosing a doctor. Most people are loath to hunt for a medical bargain. They associate high prices with quality care. Castlight, which began offers online tools that put that assumption to the test. When companies sign on, their employees can compare prices and patients’ quality rating for various providers, a boon given that individuals increasingly pay more through high-deductible plans. The rates are derived from a database of what employers have paid in the past.