I'd imagine 99% of it is perceived. I would
like to think that more expensive brushes represent more sustainably-sourced 'ingredients', better workmanship, greater attention to detail, more research behind them and overall better build quality. As the qualities of a brush are
so subjective (some like 'em floppy...
easy now!) there's going to be less of the 'research' element.
Like I said, in my opinion I reckon there's 99% perception. Typical example is a Ferrari 599 and a Ford Fiesta will both get you from A to B, but one is several hundred-fold dearer than the other.
I'm happy owning cheap brushes if I like the look of them and they do what they're supposed to!
Edit: the major difference I've seen is the adjustability of big brands, with respect to loft, hair-type, colours available, etc..