If you're patient and have a bit of common sense, you can watch ebay's prices. I say patient because you'll have to wait a week or so for a few of them to see, so you can compare what you have to what's sold. There's a huge range of quality and price. And I say to have some common sense because while ebay does represent what folks are willing to pay for a like item, it also represents what they're willing to pay when they start fighting over an item, or when they get clouded and go off the deep end.
There's other factors to add in too-is the bar included? How long is it? How much use has it seen? Does it have a full wrap handle? It's in good shape, but how does it look?
You'll see 66's go on ebay for anywhere from $500-$900+ depending on these things, and other intangeables like auction length, presentation time the auction ends and so on.
I'd pay probably $500-$600 for one. Now that's considering that I don't need a 66, and that if I was going to pay more than that ($750+), I'd be approaching the range of a pretty new if not new saw and then I'd just do that.
Jeff