It's the law of diminishing returns. Outside of very long telephotos, the first $500 to $2000 depending on the lens, normal prime to zooms buys a lot of lens. After that you start to pay for very little in the way of functional improvement. For less than a $200 I got a Tonkia 400 5.6, a used Mintola 400 F 4 is around 3 grand, that a lot of money for one stop. I have an Sigma 50 1.4 arts lens, it is really good, then again my Konica 50 1.7 is as sharp, but the Simga has somewhat better coatings and better contrast, it is work a $600 difference? (Of course it is, the Konica is of course MF, only meters in stop down mode on my Sony E bodies).