Having built a nas recently (luckily before ram/diskpocalypse) there seems to be a huge difference between making a small nas (say 4 disks) and a big one. With a small one you are essentially building a pc, and the differentiating factor is the software. Mobo, memory, case, PSU… buy consumer (cheap) stuff and you’re ok. Above that you have to start buying very expensive business oriented stuff,disk enclosure, better psu… the costs increase more that linearly , especially if you don’t live in the US and can easily get used stuff from eBay.

And that stuff is often not just more expensive, but uglier and noisier. I ended up making my own “enclosure”