Grab pc part picker and do it yourself, it's not complex.

How can there be 50% markup if the ram price is alone like 500$

And the parts aren't comparable when it comes to size and acoustics (and, frankly, aesthetics). The Steam Machine is a very small and, by all accounts, very quiet box. Might not be worth it to most people, of course, but I've seen a ton of supposed comparisons that don't take that into account at all. To say nothing of the people who claim to be able to build something cheaper but then talk about buying used parts or reusing RAM and SSDs they had purchased last years or earlier.