I've written quite a lot about collecting over the years. A good many of those posts attempt to define what collecting is (as a concept), and what makes a good collection.
I think three criteria determine the value of any collectable item:
1) Desirabilty -- how attractive the object is, and to how many people
2) Scarcity - how many of the objects exist, in relation to demand
3) Condition - how complete , undamaged, or operational the object is
Notice I didn't say old. Something can be quite new, but if it's in short supply and there's a huge demand, it can be more valuable than something very old but quite plentiful that few people care about.
If any one of the parameters change, then so does the perceived value. You can see that dynamic in action reading The Rise and Fall of 7 Fad Collectibles at Kovels.com. It's an excellent article that outlines the boom and bust of Beanie Babies, Hummel figures, Cabbage Patch Dolls, and more.
If you read the article carefully, you'll see the same themes played out again and again. When the object took off in popularity, demand far outstripped supply, sending prices up. When the manufacturers caught on and started producing the items in quantity, the market was flooded, enthusiasm waned, and values plunged.
Because in the end, a collectible item is only worth what others are willing to pay for it. So it doesn't matter if you purchased that limited edition plate for $200 when it was new. If no one's interested, it has the same value as a ten-cent plate in a yard sale. Except that you can't eat off of a limited edition collectable plate, so to many people, it's of even less value then that 10-cent Blue Willow plate.
There's a corollary to the article that I don't think the writers emphasized enough, though. Never ever buy anything labeled a collector's item. If "collectible" is printed on the packaging, the one thing you can be sure if is that the object inside isn't.