Enlarge (credit: Getty | Bill Diodato) The top 25 costliest drugs for Medicare Part D plans earned their lofty rankings largely through exorbitant price hikes—increases that, on average, more than tripled their list prices since they entered the market, according to a new analysis by AARP. For nearly all the drugs, the price hikes far outstripped the rate of inflation, with increases ranging from 20 percent to 739 percent during the drugs' lifetimes on the market.