IMHO, when you're dealing with a very common gun like a 336C or something, it makes a lot of sense to ask around about "what's it worth." That way, you don't jump on a nice looking gun for $500 when you can easily get one as good for $400 or less. But when you're dealing with scarce, really cool guns like the GT, I'd just say the value of the gun is whatever someone's willing to pay for it. (Isn't that deep?) I'd happily pay way more than $300 for a good GT, so to me, one priced at $300 seems like unbelievable luck. To others, I guess it might still seem a bit high, since the gun isn't "worth" all that much according to the books and so forth. But I'm looking like crazy for a GT, and I can't find one at any price I can manage to afford. Actually, today, I can't fine one at any price period.
(And if any of you fine folks happen to have one you're looking to unload, you can certainly read this post as a hint that I'm interested in helping you out on that.)