Well, both Waxworks and Personal Nightmare are selling at $5.99 each and those are not much newer than, say, Wastelands. Not the worst sellers, according to the store sorting. But yes, they do feel overpriced and I don't see that as a viable model for really old games.
With Infocom games there is a known way to break them into reasonable sized bundles that make sense -- at some point Activision released five separate collections: SciFi, Fantasy, Adventure, Comedy, and Mystery. HHTTG was a part of SciFi but if Planetfall is moved there from Adventure collection (where it does not belong) the bundles would be about the same size, very reasonable for $5.99 price point. Just throw in Arthur, Journey, and (of course!) LGOP can be redistributed between those bundles as well. The ideal solution would be to get Masterpieces -- and it would definitely sell at $9.99.