When it comes to some of those older titles that fetch big money, personally I think it's mostly collectible factor driving the price on those, rather than the gameplay factor. So don't get fooled by thinking more $$$ = more fun with those titles.
Yes, some of those titles play well, but for instance, compare Fathom in the $4k+ range against The Shadow in the $3500+ range. The Shadow is mainly driven by gameplay value, rather than collector's pricing. TS is not too difficult to find, but a lot of people enjoy playing it. And then compare that against Congo ($5k+) where the gameplay isn't quite as good, but they're much more difficult to find, and a bit more collectible.
So, would I like to own a fathom? Sure! Do I want to pay $4k+ for one? Oh heck no. I have more fun with other games around a similar price point, that to me, offer more bang per buck and have a higher replay value.