The question "what is it worth" is certainly open to interpretation. I could discuss that for some time and write paragraphs on it. If the USA still allowed its citizens to import military firearms there wouldn't be on functional gun left in Canada. Well, maybe one. Who would keep something valued at even $4000 here when you could sell it to an American for $25000?
If someone desires a certain firearm that is scarce to complete his collection and the only one for sale is $4000 that doesn't mean it is worth that. Your dilemma is whether you are willing to pay that.
What is an average nicely deactivated Thompson selling for in the US today? $750.00 not the $3000 and up prices asked for in Canada. But you can't import that gun. And they can't import ours. I know many Americans that will tell you their working Thompsons are "worth" $2000 but they sell for $20,000. Sometimes we forget that we live in two very different countries.
Some guns aren't really for sale (even here on CGN) they're just fishing expeditions.