The difference in Knot size is, for lack of a better way to put it, the size of the knot. The range from the 14mm Wee Scot to brushes with 30mm+ knots. A larger knot contains much more hair (28mm has 4x the hair of 14mm) and thus costs more. Personal preference is the main factor here, I don't own a brush over 18mm right now, I like them small.
Another factor is hair grade. My first badger was an Edwin Jagger Pure that was very scratchy. I then tried a Silvertip that was much too soft for me (though not floppy at all). I have settled on Simpsons Best Badger, nice and soft but still scrubby without scratching me.
Density plays a role too. I had a Vulfix brush at the $50 point that was very floppy but worked just fine. My Rooney ($75) was too dense for me and I had a hard time getting the lather out of it, but my fiance loves it.
Handle material is an aesthetic factor, but hugely influences price. Most brushes have a handle made of some sort of plastic, but brushes are also available with metal, stone, wood, bone, ivory, etc handles. The rarer or more expensive the handle, the pricier the brush.
Lastly, Brand Prestige plays a huge role. A Plisson in pure badger starts at more money than either of my two Simpsons. A Rooney in Super is around $75 for a size small where Shavemac brushes in a similar grade start higher than that.
On Price v. Performance, I wouldn't trade my "Low End" Simpsons brushes for anything.
I hope that pretty much clears it up for you. Happy Shaving.