In this price range i would get something from Arctic cooling or Spire. Or you could try and buy a stock heatsink from someone who doesn't need his anymore.
$15 is just about the price at which hsf start (don't get anything for less!). Good hsf are available from $25.
If you want to replace your stock hsf with something better you will need to spend a little more. If it's broken and you don't mind noise and don't want to overclock get something from Arctic Cooling or Spire.