The closest I could come was a friend's half dozen year old Blue Book, and for one of those guns in 100% condition they valued it at $1500.
Balance that against a collector I spoke with over a decade ago, who said one of those guns FACTORY CHAMBERED in .223 would fetch as much as $5K.
And that 99.99% of 600 series rifles in .223 are rechambered .222s.
Gunbroker is often mentioned as a source for what something is worth, but it is important to remember that Gunbroker is an AUCTION.
And, that people can, and do ask ridiculous prices for things as the opening bid.
Someone may list a gun for $4k but that doesn't mean its worth that. That gun might sit there for YEARS waiting for someone willing to pay that price. It may never sell at the asking price.
Until and unless someone actually buys it for the inflated asking price, its NOT worth what they want for it.
Yes, the factory .223s are uber rare, only a couple hundred made, but that doesn't mean they are hugely valuable, their value will be determined by market DEMAND, and if there is little or no collector demand, the value remains low even if only a handful were ever made.
Now, if/when there is a demand, the number made WILL affect the value, but, not until then.
Remember, just because they stopped making something does NOT automatically make it valuable. There's always a reason production ends, and its usually due to lack of sales (buyer interest).
If you can find a well heeled fellow with a passion for Rem 600s, they may pay a premium to get a rare one. Other than that, its worth what you can sell it for, which often isn't what sellers THINK the gun is worth.
I've got half a dozen of the 600 series rifles, .222, .243, 6mm, .308 and .350. Would like a 6.5 but won't pay what's being asked for one these days. And forget me even considering a .223 at a whopper of a price. Its rare, but that doesn't matter to me. And, I think, I'm in the majority in that regard.
Blue book said $1500 (for pristine) half a dozen years ago, so that price or even a bit more today wouldn't be all that far out of line, but 4 or 5 Thousand? No barking way, for me, at any rate.
Good Luck!