Oh come on, Toms Hardware did a roundup of desktop SSDs a few months ago. The iMac drives beat all but one of them, the Sabrent Rocket and that comes in at £750 for a 4TB card.
So sure Apple storage is pricey, and if you’re willing to compromise on performance you can get something that looks equivalent for a lot less, or something actually equivalent for a bit less. But there are a lot of other things about Apple gear you can’t get anywhere else full stop, at any price.
According to who? Everybody cares about value for money. Specs aren't the only reason a product is valuable though, of course. I pay a mark up on Apple gear because of the value I get from the software and customer service, but I still need effective tools and that means up to scratch spec-wise.
So sure Apple storage is pricey, and if you’re willing to compromise on performance you can get something that looks equivalent for a lot less, or something actually equivalent for a bit less. But there are a lot of other things about Apple gear you can’t get anywhere else full stop, at any price.