Most I've ever spent on a knife was around $850.
I think there's more to a knife than price though. At the >$500 range you are usually getting into blades that should probably be collected. That's not to say you can't use them but at that point you are paying for premium materials and scales and probably the name of the person who made it.
Possibly two of the most expensive folders I've owned by big name custom makers were not as dependable or as well made as some of my favorite production knives.
Fixed blades are hard to judge because there are not as many details that need to be dead on to make a great fixed blade. Really all that needs to be judged is steel, heat treat and blade geometry, all things being equal in this area and you're just paying for name and style.
In a folder you have about twice as many aspects that need to be considered when evaluating quality. These rules change again when looking at slip joints vs. locking folders...
There are several high end production knives that will compete with custom knives in most aspects, but they will never have the hand made character of a custom.
In my experience, there is not a production slip joint made that rivals even an inexpesive custom. To me this is where the biggest bang for the buck improvement in quality is in the knife world. In most cases a $250 custom slip joint is 3-4x's better than a $75-$100 production piece.
Sorry for the ramble but I have a lot of thoughts on this matter
