What will a gallon of milk set you back? How about a new car? You probably have a rough idea. But what about a medical device — the kind that gets implanted during a knee or hip replacement? Chances are you have no clue. And you are not alone: The surgeons who implant those devices probably don't know either, a just-published survey shows. Medicare spends about $20 billion each year on implantable medical devices — nearly half of it for orthopedic procedures. And as the population ages and more Americans get joint replacements, that number is only going up, which will have a bigger and bigger impact on the nation's health care spending.