You can get useful fixed blades from $35 up. At the low end are interesting designs like the Gerber Epic - a 3 1/2" wharncliff with a bottle opener in the handle, the sheath has a pocket clip for front carry.
Next up would be deer hunting and utility knife patterns up to $125, plus military. From there to the $250 level you can find all sorts of interesting, useful, and certainly outrageous stuff. More than that is spending money to get small incremental improvements, hand finishing, a name Brand, exotic steels. High alloy steels are much harder to work and finish in large knives, the labor goes up exponentially. In a large field knife, it's arguably more money that might be needed in a knife.
An expensive knife is just that, nearly heirloom, but you only get one for the money. It could buy half a dozen other knives that are serviceable, and over the course of years, someone could find exactly what features they prefer and would like to have in one. There's nothing wrong with knives under $100 - the money saved could be spent for other gear.
As time has gone by, my fixed blades have become smaller, less exotic, and plainer. Fashion and fads have come and gone, a fixed blade boils down to a handle that is comfortable to hold, well rounded, and reversible for other grips. Blades have gone from heavy tactical swedge grinds to simple flat grinds, maybe with a finger choil to choke up with, and a bit more width than older ones. Grips have gone from stag, wood, leather, rubber compounds, and now plain canvas micarta or G10. The sheath has become a much more important item, and the steel in a 4-6 inch knife a lot less so. Bigger knives don't require a better steel to get good performance, their grind and edge bevel is more important.
Because of that I've gone from using Gerber Mk II's or Randall #14's to the Benchmade Nimravus, and now looking at the ESEE Laser Strike. A fixed blade is just that - not a folding one, and the added inch or two of edge isn't all that important. What is important is that it can take a lot more leverage for hard chores, and even do some light chopping when a hawk or axe isn't in hand. It can be a sharpened pry bar with a lot less probability of snapping off the tip.
But, no everyone buys a fixed blade for the same reasons, which is why there are so many designs, and why we ask, What will you do with it? Like any tool, it has a different shape for a different purpose. What has to be disregarded on the internet are all the responses where someone promotes his favorite design when you don't really even know if you need it - like having someone recommend a drilldriver with phillips bit when the job really needs a star socket on a ratchet. One is for installing sheet rock, the other for taking off brake calipers. Both jobs are all about turning a fastener, but they aren't the same at all. Neither are knives.
It's not just about cutting, but what and where you do it.
