Costco's deal to replace American Express with Visa as its exclusive credit-card company highlights an economic principle that should surprise no one.
Consumers benefit when a business uses its market power to negotiate lower prices and passes along the savings to customers.
Conservatives have championed such market forces for decades, arguing that if government regulators just got out of the way of businesses, consumers would be the big winners.
That is, unless we're talking about drug prices.
Medicare, the federal healthcare program, is prohibited by law from haggling with makers of prescription drugs over the prices paid by its 54 million...