CalamityRanger: Very true. That's why I prefer Ebay over say, Amazon for my older goodies. It's usually much easier to spot the sellers that are using entirely reasonable prices. For the most part I won't go over $50 for the things I grew up on, but it really matters just what the asking price is and the condition (it kills me to see older games being sold just in a cd sleeve or jewel case for $50+. I recently bought the disked version of the Dagger of Amon Ra, all the goodies were in great condition and I only paid $50 for it so it was worth it).
A few years back I was looking to gather some of the Infocom games and their feelies, most definitely A Mind Forever Voyaging. I put it off for a long time though since all the asking prices were way more than I personally wanted to spend. I'll just have to keep my eye out for a better price range if it ever comes.
I will say that the Infocom boxes are one of the few examples of old games I'm comfortable paying $50 for (and I prefer paying less, definitely).
If you're not particularly adamant about sealed boxes or complete contents, it can also be pretty cheap. For instance, I just picked up a copy of Infocom's Stationfall in great shape for $10 because it was missing the Stellar Patrol sew-on patch (one of the more commonly missing items). All documentation and other props were there right down to the Infocom "Passport" advertising catalog.
Likewise, I got a complete copy of The Witness (which has incredible cover art) for $12 because someone at some point had stacked something on top of the box which left a dent in the back of the box.
Of course, both of those purchases weren't made on eBay or Amazon, but from a storefront run by another collector who is very fair with his pricing.
Oh, and good luck with the hunt for AMFV. For whatever reason, it's one of the harder to come by, not that Infocom games are exactly easy to find anyway, but...AMFV is one I'm still looking for myself.
As for newer older games....If you happen to be anywhere near a Half-Price Books, I've found some fantastic things there really cheap, including some fairly rare stuff like the boxed DK mentioned earlier for around $8.