I'm lucky enough to have picked up three complete, unbuilt Sealab kits over the years, thankfully before the word on that kit got out, and I still can't bring myself to use anything other than my own castings of the parts I need for any given pattern or model.
The problem with a kit being worth $500 today, for example, is that if it contains only one of a part that you need for a particular pattern or model build, then the replacement cost of that single part is $500. It's not $500 divided by the total number of parts in the kit, but $500 to replace that singleton part because you have to buy a whole new kit just to get that one part. And that's assuming the price of the kit doesn't increase, which it almost always does.
And the other problem with these rare kits is that I change my mind about future builds just about every year. What if I really do want to build that five-foot Falcon five years from now? I can't use up all of these parts with crazy replacement costs on this project or that project. It was difficult and expensive enough collecting all of these rare, vintage kits the first time around!