alt Hacker NewsThe steam machine has upgradable storage and RAM, both being their respective commodity connections (m.2 & sodimm respectively). There's even 1 sodimm slot free from the factory if you want to immediately plunk in a 2nd 16gb stick.
Those two are realistically the only upgrades someone buying a prebuilt instead of DIY is going to entertain doing regardless.
But that's all besides the point, which it's simply that clearly the steam machine is priced fairly for the hardware it contains in the current economic market. Whether or not you personally would prefer a prebuilt or to DIY is entirely irrelevant
Note that some machines will have two 8GB sticks, others just one 16GB stick. This is mentioned in the Gamers Nexus interview with some Valve employees, who were talking about the difficulty of finding RAM at any price. They had intended them all to be two sticks, but some will come with one because that's all they could source.