I have a pair of these on my 500. I think they look great. Now to your question. What kind of shape are they in? Do they need any work? If they're in immaculate condition, then I'd wouldn't take any less than $200, but that's me. You'll need to find the right buyer who knows what they're looking for.
+1 As with most things, they are worth exactly what someone is willing to pay for them. Pick a high number and start lowering until someone bites.