i dont think Valve will produce the box... they will set some rules on what can be "licensed" (for free) as "SteamOS Powered", with some minimum configuration.
However the pricing is up to hardware partners. And they want to make standard margins for these. So take away $50-$80 from Dell desktops to see what the pricing could be. For instance, base Alienware X51 looks like could be easily transferred to SteamBox with $599 price for model with i3, GTX 645 and 6GB of RAM. Maybe use 4GB, and AMD APU to get some lower price?
In any case, nobody is going to be subsidizing the cost or selling at the cost. All the hardware makers will want to sell at standard retail prices.
Latest Tom's best configs for $650:
http://www.tomshardware.com/reviews/build-a-pc-fx-6300-overclocking,3617.html
Yep. It's basically looking like the Blu-Ray player business if you wanna compare it to other TV devices.