$400 is for the empty enclosure and $1,000 is for the enclosure with a NIC in it. Obviously it's still a ripoff because a 25G NIC does not cost $600.

Yeah the best option for cost is to buy one of the many TB4 to PCIe adapters on the market now (I've tried two), that are less than $100. They require external PSU power though, and they don't look nearly as nice on a desk.