there have been lenses similar to this from companies like Vivitar or Phoenix or Samyang ( or something like that ) for a long time, ranging from 600mm - 1000mm+. All for just a few hundred dollars.
These are basically telescopes. The concensus of people who have tried it is "Don't bother - its a waste of money". Poor contrast, poor resolution, washed out colors, and unuseable unless under the brightest light. Better to spend some money on a real 400/500mm lens. The focal length is nowhere as good, but the results are loads better.