This idea is what the collectibles market is built on, and why absolutely nothing made that is termed a "collectible" by the comic book/card game/pocket monster companies is going to be worth anything in the long run.
This is true of the collectibles market in general and not just of gaming/nerd collectibles.
Generally real collectibles must be:
a) Rare or even unique. There must never have been very many of them.
b) Not recognized as being valuable at the time that they are first created thereby reducing even further the availability of the item.
c) Have significant cultural or historical value.
If you look at for example, base ball cards, the ones that have any value at all meet all these qualifications. The were printed in small print runs and included as rewards in packs of cigerettes or chewing gum. Most people considered them disposable at the time they were printed, and the valuable ones are of players which went on to have a monumental legacy in what was then 'America's Favorite Pasttime' before it was understood just how big professional sports and sports stars would become.
No modern baseball card meets any of those qualifications.
The other thing to watch out for in gaming collectibles in particular is confusing the collectible value of the item with the items utility. Game peices with a large impact on a game can achieve high demand for players of that game. But that's not the same as having value as a collectible. The true mark of a collectible is that you could make a four million copies without effecting the value of the 'real' one in any way. Reprinting the OD&D 3 volumn set with a digital signature of Gygax and Arneson wouldn't effect the value of a real one. Giving away PDF copies of the 1e Tomb of Horrors wouldn't in the slightest effect the value of, for example, Gygax's original hand written or type written notes for the Dungeon or hand drawn map. On the other hand, printing 10 or 12 million Black Lotuses would by my estimation drive down the value of the originals by about 80%. An Alpha Block Lotus would still retain signficant value as a collectible, but without the artificial constaint on their availablity as game peices, they'd be much less valueable. I base this estimate on the overall price trends of MtG cards over the last two decades or so.
Many GW's minatures have done a rather good job of retaining their value, but they do so primarily as useful playing peices that are often hard to come by. Little if any of the value is their value as collectibles because they don't meet any of the three qualifications. The same is true of many other 'rare' game peices - from Ogre minatures to rare 80's RPG books. They may be expensive to obtain, but don't necessarily assume they are collectible and investments. That's why I laugh when people try to add value to a miniature because it is 'rare'. And 3D printing may ultimately crash even that value.