If you pay for a speaker with the price of $100-200, then yes, sure, you're not gonna get something great or very ideal for reliable mixing or mastering. They have ports, transient response is probably not that great, etc, etc. But when you spend more than $4000 (and that's probably pushing it) for a pair of speakers, I genuinely believe it's not that worth it. maybe unless it's in the Neumann category and you're just rich. Or maybe if your studio needs very big mains because you want to impress clients or whatever.
most mixing or mastering engineers don't need to break the bank to get a good mix or even be able to master. Engineering has its limits and nothing man-made is perfect, let's be honest. It can get really good, but there will never be "perfect". Objectively, perfect doesn't exist in the Audio world. Its all taste. Those super expensive Neumanns or Genelecs aren't perfect either, but they are very good.
My point is that for most mixing or mastering engineers, even if you have the money, it is unnecessary to spend a fortune on the best Genelecs, ATCs, and honestly, probably even a KH310.
I really think the price of all these speakers comes down to marketing, and sure, the additional benefits the monitors could provide, or how they were engineered. Like the KH310 for example, is a really great piece of gear ill admit that. but for most speakers its the marketing due to the brand name which makes the price so high. For example, why waste $8k on a pair of Amphion Two 18s? or $13K for ATCS? Those prices are nuts. The engineers you see using those Amphions speakers, they didn't buy them. They were given to them by those brands as a product for advertising. Not saying the speakers are bad and such, but the price is a massive stretch and you can get great results for less.