Telephoto lenses aren't cheap, and there's no reason to waste money on a new one if you have an older one lying around. With a few modifications, you can fit an old telephoto lens on a new DSLR camera.
This isn't a simple duct tape hack, but if you have a bit of patience and a steady hand, you can save quite a bit of money. You'll need to make sure there aren't any scratches on any of your lenses, and you'll need to do some pretty precise measuring (to get the camera-lens distance just right), but you won't need any tools beyond a few small screwdrivers, either. You'll probably have to purchase an adaptor to correctly mount your old lens, but again, it'll be a whole lot cheaper than buying a new one. Hit the link to read the full instructions.