Some people carry the cheapest knife they can get in the event it gets broken, stolen, or needs to be discarded (fill in the blanks here). Cheap knives tend to get broken more easily, if they're folders. Some of the non-folding knives out there that are cheap lack a full tang necessary for good, solid function.
Collecters often don't care that much about price...and will spend hundreds...even a thousand or more...for a knife. They'll actually carry some of these.
I bought a Strider Valkyrie for close to $800. I love it. Its huge, though, and difficult to carry. Two friends of mine carry Strider SnG's...each cost around $400. One of them also carrys a Strider Kerambit as a backup to the SnG and his Glock (he's a cop here in Bloomington). The Kerambit runs close to $400 as well.
If you intend to use the knife as a tool as well as a self defense weapon, it might behoove you to get a robust and expensive one. Striders are good for that, as they're highly over-engineered. If you're just going to use it as a box cutter, though, a cheapie will do.
And when it comes to self defense, a cheap knife is better than no knife.
Regards,
Steve Scott