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.