I think it has to do with preference rather than affordability. One of my clients is a 40-something corporate litigation lawyer, head of one of the largest firms in Texas. Needless to say he could afford whatever sunglasses he wants, but I've only ever seen him wearing Wayfarers, which are relatively cheap. I find the same thing with very rich people and cars, by conservative estimates I think he must pull in 75-100 million/year, but his regular car is always just a current-year S-class. He could drive a new Bentley or Rolls or whatever he wanted, but I don't think he really cares that much. Now get him talking about golf, well he'll spend $40k on some golf gear inside half an hour. Passion.
I personally just tend to lose the damned things, lol. I can afford to spend maybe $75-$150 on a pair every couple of months without noticing at all, so I'm not rich or poor, but I've noticed the quality of cheapies has gotten much much much better over the years, so I'm not really concerned about brand names at this point. I have nobody to impress other than my eyes I like the feeling of losing the pair and completely not caring. There are other items which I am much more picky about, and am willing to spend a premium on, but sunglasses just aren't one of those items. Ditto watches, I love cool watches, and some of the brands are badass, I have a Tag that's a good one for the price, and an awesome Vollmer, but some of the best watches I've ever gotten have been $10 understated knockoffs from Canal St, which sometimes last for years and years, despite being made with slave labor in some Chinese hell.