I have the Sigma 50-500 on my (Nikon Dx format)
I think I paid $1,000 or $1,500, I have had it for a while.
I do not think it is available any longer - not sure.
It is NOT a F2.8 but then again it was not $6,000
I love it, some complain about "creep" but I have never had an issue.
The eagles on my site were taken with it and with a 1.5 teleconverter.