Any Anaconda in Very good condition would be worth $500 with out a doubt.
The 45LC version is a little tougher to find than the 44 mag. The 4 inch tubed 45LC is a lot tougher to find.
Both 6 inchers are worth about the same from what I've seen.
I know if I found another Anaconda in nice shape for 5 bones, I would own it.
Don't forget, Colt is NOT makin' them anymore.
That's a good price. Not only are the .45s harder to find (they didn't make as many of them) they seem to be sought after by more people now. The .44s are better shooters IMO unless you want to do some good handloading. My range buddy has a .44 and it's a tack driver with just about any factory load except the light bullets. I have the .45s and although it shoots decently with factory loads (mainly jacketed bullet loads) it starts to shine when I use oversized cast bullets since it has the typical oversized throats Colt is known for.
I just sold my 6" .45 Anaconda recently to finance another purchase. It was by far the most accurate .45 I've ever had. Out of the box would shoot circles around my Blackhawk, which has had a good deal of work done to it. I never shot factory loads in it but with a Laser cast 255gr. hard cast and either 10gr Unique or 18.5gr #2400 (don't use this load in Colt SAAs or clones) it was a tackdriver. For $500 I would be all over it.