I bought brand new 10 Gbps NICs ($47/pcs) and switch ($120) for less than that and replacements are readily available. The 20m AOC cables were indeed $40.
A NAS home build of this size will never exceed 10 Gbps, I barely get ~ 2 Gbps out of the spinning disks.
the main application where one will see real 10Gbps speeds is if the NAS also has a fast NVME SSD used for smaller file size needs at high speeds...
for instance I have a setup which is meant for working with uncompressed raw yuv420 or yuv422p 1080p and 4K video, there's a 512GB NVME SSD and a 1TB SSD set up as individual JBOD and exposed to the network for video editing scratch file storage, and it will definitely saturate 10GbE.
this is actually needlessly complicated, if/when I build a more powerful desktop pc again I'm just going to put the same work file space storage on a 2TB NVME SSD directly stuck into the motherboard.
Very valid point, but I don't know how fast you will hit the chipset speed limits; that NIC is connected to the chipset that is connected to the CPU via a 4x PCIe link and from there to the nVME with 4x PCIe link. In theory you have 4 or 8 GB/sec max bandwidth, but the CPU-chipset link is not dedicated. If you go for Threadripper the math is very different, you have lots of direct CPU connections for (multiple) NIC and multiple nVME.
you may be referencing the original post's cpu/motherboard combo which is not the same as the pci-e bus/lanes, slot and setup on my home file server and desktop PC.
A NAS home build of this size will never exceed 10 Gbps, I barely get ~ 2 Gbps out of the spinning disks.