I saw a Neiman-Marcus ad showing a male model wearing very ordinary looking casual clothes. The cost of the outfit was $650!
Frankly, the pic could have been one from Target, PC Penney or Sears which would have cost less than a $100. Why is that?
The fabric is often one reason. Pure silk (a man's $2,000 suit is likely to be silk, I think, or wool or whatever), high grade cotton, 100% linen, etc.
Quality of manufacturing. Seams are generous, well made, all facings are present, lined, etc.
Name brand. You pay UP for a name brand. Sometimes because it's trendy, sometimes because others will recognize the brand and know you're rich (like a Coco Chanel suit), and sometimes because the name means quality (like Ralph Lauren).
Fit. Well made, expensive clothes often fit better (but not always).
Taste and style. Name brands have a certain style that you can count on. Classy and classic - Ralph Lauren. Clean, streamlined, not trendy - some other name brands. Very trendy but chic - some other brands. Eco friendly, animal friendly, and cool and flattering - McCartney.
Up to date: If you buy an expensive name brand one year, you know it's in style that year (the style or the color or whatever).
I have bought some Ralph Lauren pieces on sale. They wear very well and last forever. Truly high quality. While some jeans, for example, I got at Target, fit well and look good, but then stretch so much that they almost literally fall off. Something very wrong with them.
The cheaper clothes, OTOH - you take your chances. Still, I've bought cheap (er...I mean inexpensive) clothes that look high quality and wear very well, and are decent quality. And I've bought some pricier name brands that got misshapen after washing and were a waste of money.
If you look at the stitching of cheaper clothes, and look at the inside of the piece, you will probably find uneven stitches, smaller seams, maybe some errors in sewing, facing sewn in backwards sometimes. Things like that. That may not matter to the wearer, though.