I've had a lot of experience with collectibles, and people's gut instinct is usually all wrong. Very expensive things rarely appreciate in value, and to get one, you have so much invested, you can't sell without losing money. The place where money can be made is inexpensive things that are extremely popular, yet disposable. Buy a large batch and hang onto them for 20 years, and you'll be able to triple your investment or more. It's a lot easier for something worth $20 to triple in value than something worth $500. Imagine if you did that with 5 cent packages of baseball cards or comic books back in the 50s and 60s... you would be rich today.
For instance... if I had to predict the future of knife collecting, I would probably go against what most people here tend to think. I wouldn't invest in custom knives or high end GEC. If I was in my 20s, I would look for specific models of inexpensive knife models made by companies like Spyderco, Gerber, Rough Ryder and Marble's that are highly sought after when they are initially released. I'd sock away a few dozen of each and keep them mint in box. Fill a closet shelf with them. Right now, these cheap knives are considered ephemeral, but nearly everyone has one in their pocket. In 20 years, people will be looking back fondly on the knives of their youth and will want one just like they used to have, just like we do about the knives our dads gave us. It is pretty much a certainty that in the future high quality knives will cost a lot more than they do right now. You should be able to buy a knife today for $15 and sell it for $75 in 20 years. Even if they don't appreciate in value at all, it shouldn't be hard to sell them for what you paid for them.
So go invest in cheap knives by the crate load and become a tycoon of the future!