Trying to say this as simply as possible...
There are several factors in how fast anything will appreciate in value. Rarity and condition are two prime factors but perhaps even more is desirability. You could have an object that is one of a kind in perfect condition but if no one wants it it has no value.
You are talking about Case knives in this thread so let me use a few examples. The Case knives that are being sold for $50 to $60 are made in huge numbers, this cuts back on the rarity of the knife. You can keep it NIB but so will a lot of other people who collect Case knives so not to many people want one, this cuts desirability. As a result the value increases very slowly, I would say for one of these common Cases it's value will only increase at the same rate as the retail price for a new knife from year to year.
Now if you had bought a Tony Bose Case things change. These are made in far smaller numbers thus increasing it's rarity. From just this forum you can see they are very desirable and keeping on NIB is no trick...just don't use it. This knife will increase in value at a faster rate than the $50 Case.
Some knives jump in value when the are discontinued but this is not a sure thing either, sometimes being discontinued makes the knife drop in value. I don't expect to ever see my collection going up in value faster than inflation, at best they are a long term investment...I would say on the lines of 30 to 50 years before the appreciate greatly. Of course sometime you get surprised when you buy just the right knife but that doesn't happen very often and almost never with a $50 knife. If my collection is kept together and keep in good condition I think my great grand kids might be happy but I will never see it.
Buy knives because you like them, care for them keeping them free from rust and harm, feed them with mineral oil, give them pet names....but don't expect them to make you rich...lol. If you want knives that will quickly rise in value plan on spending 10 times as much or more than you will on that $50 Case. Knifes and guns are about the worst investment there are for short term gains but there is a chance you can make someone happy three generations from