Seems to me the criteria of interest would be: fit, finish, materials used, personal appeal of the design and maybe suitability for use.
Given the price range you quote, you'll see variations in all these attributes.
Fit will go from obvious and inconsistent gaps between handle material and fittings to consistent lines or no apparent transitions. Grinds will go from not centred, to perfectly so. Grind lines will show some variation side to side, or be completely symetrical. The knife will come with a loose, generic sheath at the low end to a tight, secure sheath in the better examples. The sheath may be kydex or leather.
Finish is probably the the area for the greatest variation. There are machine applied finishes and hand finishes, and these may be used in combination on the same knife. Machine applied blade finishes are straight from the belt (usually parallel lines), bead blasted (funtional but ugh!), "stonewashed" (comes from tumble deburring technology that's been used for eons in machine shops to deburr parts cheaply, remember this - cheaply)) or buffer generated satin or mirror finishes. You can look for consistency in any of these finishes and if they have reduced the crispness of the grind lines. Always interesting to look for deep scratches and steel grain in the buffer applied finishes. Both conditions are indicative of skipped steps in the operations prior to buffing. Look for scratches and hollows in the handle material (usually buffed over). Look for scratches in the ends of the pins securing the scales (if present). The hand applied fifish is hand rubbed, generally parallel to the edge. The higher end will typically have finer grit finishes, no start and stop marks, swirls, or wiggly lines.
As far as materials used: blade steel is an obvious consideration and really depends on the tastes of the maker and you. You do need to expect to pay more for Talonite or Stellite. The handle is where you will encounter lots of variation. Micarta is tough, stag is pretty, so are the stabilised woods. Natural wood has charm of it's own, but as Les and others say, there are resale considerations. Pakkawood/Dymondwood may appear at the low end. It's a practical choice, but tends to be hideous. The monochrome variants, particularly cocobolo are nicest. Pricewise, you'll likely see this progression: Pakkawood, natural wood, buffalo horn, micarta, and then stag and the stabilised woods. Ramshorn, and fossil ivory may be out of this price range.
As far as personal appeal of the design is concerned - that's up to you. I would recommend however, that you get a copy of "The Cutting Edge" from AG Russell. Whenever I am at a show I pick up a copy from Paul Basche and cheque out the black and white ads. There are BUCKETSFULL of ugly knives for sale on the secondary market. The important lesson here is that someone must have liked them well enough at one time. So will the knife maintain its appeal? Personally, I think that a Loveless dropped hunter is a real work of functional modern art. But in yellow linen micarta from 1971, even it looks like a toy.
Suitability for use is tough to judge, particularly if you won't be using the knife. I suppose if it comes with a shoulder harness or a sawback it would be less usefull. Part of the ongoing popularity of the Loveless drop point is that it is a completely practical design (not to discount all the hype and hoopla over the past 20-30 years).
Seems to me that at the low end of the price range functionality should be the primary consideration with materials and execution becoming increasingly important as you move up the range.
Sorry for rambling, and of course this is all very much IMHO (and reflects my own tastes and style I suppose).
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