I think I know what you are talking about but $15 is just for a 70w HPS bulb, you need and socket and a ballast that are made for these types of bulbs .
Most people that buy their stuff from lowes buys the $50 floodlight that has a 70w HPS or you can go $80 for the 150w HPS floodlight. The Floodlights have the right socket and the right ballast that light these diff bulbs .