I don't buy super expensive, custom or rare knives as investments.
One of the things I enjoy about collecting knives is that they hold their value better than a lot of other hobbies! I don't expect to MAKE money, but I don't expect my collection to ever become worthless or be worth only pennies on the dollar, either. Worst case, I figure I can get back at least 50% of the money I spend on my collection, minus ones that I use heavily. In which case, I've gotten my money's worth from the use of them - not the sale. And possibly as much as 90%.
While some people have made money on other hobbies, including the ones I'm going to mention, many people have lost a ton of money on sports cards, NASCAR collectibles, boats, music collections, movie collections. Ever try to sell a CD that you paid $12 for? or a DVD that you paid $20?
I know anything is possible, but I would be shocked and disappointed if I ever had to cash out my knife collection for less than 50% of what I've invested in it. I believe that if I cashed it out today, that figure would be between 75% and 90%.
Of course, your results will depend largely on two things: 1. how wisely you buy 2. the market conditions when you sell.