I do not fully aggree with what most say, stating that it basically is luxury.
imo it is not, and I will explain why. The first expensive knife I bought was a benchmade 51, wich was and is great. After that I bought a benchmade 470 emissary, a benchmade 940, a benchmade 62, a sebenza and a sng cc. I still carry my 470 sometimes because it looks awesome and it takes an amazing edge, comparable to that of a sebenza. So indeed it does fulfill the strict purpose of a knife, cutting. However, when I open the knife, i can wiggle the blade and the handles will come aside just a tiny bit, enough to move the blade a tiny tiny bit (this is not blade play, because the pivot is perfectly set, but just the overall construction).
I can do the same with my 940. Is this bad ? well, if you just want to cut, then no, thats no problem at all. But we dont use knives only to cut, you'll often get into situations where you have to do a bit of leverage or something like that. My sebenza and my strider dont budge, they are rock solid and feel like bank vaults.
What is true is that price is often determined by other factors than toughness etc. For example craftmanship plays a big role. The sebenza really is a piece of art, the design and how the design is put together is amazing. ditto with the mnandi, its a small knife, it's not tough, but it's probably one of the most beautiful knives ever made. The strider on the other hand, is not an exercise in craftmanship and beauty, but in thoughness, I would stab it in a brick wall knowing it wont budge.
lastly, also the materials used and thusly the production costs play a role. My 470 and 940 are made out of aluminium while the sebenza and the strider are made of titanium (and g10).