Powerhouse rare? You realize that such rares like Lost Tamoachan and Ghost Tower of Inverness were created in the hundreds, not in the thousands that the mods you are talking about have been released. Maybe in 25 years they will be worth a hundred or so, but, that's about the extent I think they'll ever reach. And that's probably being generous.

You also have to understand that a lot of the value of the true rares is derived from nostalgia, at least IMHO. I'll be interested to see where the values go when the Grognards start dying off, and the new people who might want to collect don't have the emotional attachment to the source material like we do.

Zen

Some of these 4th edition store only promo releases do pretty well on Ebay.  The Lair Assault not so much as the Encounters series though.  Forge of the Dawn Titan and Talon of Umberlee have already cooled off and I expect Tyrantclaw will do the same soon enough.  But 20 years down the road who knows?

Speaking of the Encounters series, has anyone checked out some of the recent prices on the newer releases?  The Elder Elemental Eye and Web of the Spider Queen are going a lot higher than some of the earlier releases.  Most of those I dont recall ever topping $100.  Six copies of Spider Queen recently sold for over $100 with three of them going for over $150.  

There have been a ton of these store promo modules ever since 3rd edition started.  I was collecting these for a while and they have the typical pattern of selling for large prices as soon as they are released and then cooling off considerably.  WotC seems to have a "special release" for almost every major book they release, free RPG day, and gameday on top of the various instore gaming series.

A number of the 1st edition tie-ins seem to have held their value well but many of the others sell for under $10.  Whether they become "valuable" later on really depends on who decides they want to collect D20 and 4e stuff.  Since these games are heavily based on the miniatures lines, it is probably someone who will also collect miniatures.

Also, most of these items came with a set of instructions, minis, battlemats, tokens, characters sheets, etc on top of the actual module itself.  Finding "complete" copies with everything is extremely difficult - I think these will be the ones that become collectible selling for a premium but the module by itself will probably stay under the $25 range.

One of the biggest problems is that although these are relatively new, I have yet to find a website that catalogs all of them reliably and states what should be included.  This means that they are difficult for collectors to search for them and probably one of the reasons most sell for low prices (the other reason being, I don't think many people collect them).

The Adventurers Guild series is probably the closest thing to these lines and they seem to sell in the $30-$50 range.  However, the AG modules are much rarer and sought after by TSR collectors.

I'd be interested. I haven't even started going after promo's or the encounter modules mainly since I have no clue of what's out there.