There's a certain amount of overheating in the market because of people buying speculatively, hoping that a deck will go up in value by some order of magnitude. In the end, any deck is worth exactly what someone is willing to pay for it, no more, no less. This makes valuable a very relative thing - which in reality, it is. I could own a bottle cap collection with all sorts of rare specimens in it, and in the minds of non-collectors, it's worth nothing more than its recycling value as scrap metal. But to a fellow bottle cap collector, it could be priceless.
The best you could do to determine future value is determine what the vox populi is regarding it. Rarity alone doesn't make a deck valuable. There are countless "only 5000 made" decks on the market, and they trade at wildly different values in the after-market. The ones that people rave about because of beauty, handling, longevity, etc. are the ones that generally go up in value a lot, and the ones that people deride for the same reasons are the ones that stagnate in value, not rising much at all - or even doing DOWN in value.
These values can change over time, especially if the deck becomes more rare or difficult to find. Remember, a deck can be made in a run of 5000, but how many of those decks are kept in hermetically-sealed museum glass, and how many are actually seeing use and eventually wearing out? But the fact remains that only time will determine that factor.
Here's a caveat for you: remember when the "Death of Superman" comic book came out, and how so many people bought it at exorbitant prices because they thought it would become so much more valuable over time? That was right before the time the bottom fell out of the comic book market - those who bought it as some kind of investment for their retirement were sadly disappointed. But those who just plain read it hopefully got some entertainment value out of it. The supposed rarity of it was canceled out by the overheated value of the marketplace and the fact that most of the books issued were kept by collectors. "Action Comics #1" is worth what it's worth because it was made at a time that comics weren't thought of as collectibles (much like other ephemera like playing cards) and as such very few survive today - in addition to the fact that it's the first appearance in print of a beloved superhero character, perhaps the first superhero.