I wonder what it would go for on eBay? Right now someone is trying to sell a Sigma 300-800 for Canon and has listed it a couple of times.
Myself, I find that a 30D or 40D, 1.6x factor, with a Sigma 120-300 f2.8 or Canon 100-400 f5.6 IS, and 2x get me enough reach and I can actually hand hold them. Although a BushHawk shoulder stock makes holding them nicer.
The Sigma with a Sigma 2x TC gets me 960mm effective at f5.6. Don't think about trying any other 2x TC, it is too soft.
The Canon with a Canon 2x TC gets me 1280mm effective at f11. I have taped the contacts on the Canon 2x TC so the camera still attempts to auto focus. (
www.allensphotoblog.com/blog1/2006/12/the_old_tape_trick.html)
Here in the sunny Finger Lakes of New York state, f5.6 is usable, f11 really isn't.
And Sigma now has the 200-500 f2.8 zoom listed on their web site.