Valve willing to sell at a loss
I don't think that Valve will sell the Steam Machine at a loss.
Closed-system console vendors often do, then jack up the prices of their games and make their money back as people buy games. So why not Valve?
Two reasons.
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They sell an open system. If Valve sells a mini-PC below cost, then a number of people will just buy the thing and use it as a generic mini-PC, which doesn't make them anything. A Nintendo Switch, in contrast, isn't very appealing for anything than running games purchased from Nintendo.
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They don't have a practical way to charge more for games for just Steam Machine users
their model is agnostic to what device you run a purchased game on. So even if they were going to do that, it'd force them to price games non-optimally for non-Steam-Machine users, charge more than would be ideal from Valve's standpoint.