looking for cheap full-frame lenses

I got a Nikon D800 for Christmas, but all my existing lenses are DX format. I'm looking to get at least one versatile, general purpose zoom that's full-frame, auto-focus compatible, and under $500. Any suggestions? I'd really wanted the Tamron AF 18-200mm f/3.5-6.3 XR Di II LD Aspherical (IF) Macro until I realized it wasn't full-frame. I can't seem to find something equivalent.