Comparing similar specs from PC builder sites and just from Newegg/Amazon, a similarly specked custom PC to the Steam Cube will be $900 to $1000. So if Valve can get at or under that price it will probably be a good deal.
My current PC is pretty similar specs too and can run demanding games like Darktide or Helldivers 2 at pretty alright settings just fine so, if they can make a good deal for PC gaming it’ll be great.
@arc valve said it isn't a console, so it will probably be about $900. I'd be shocked if it was cheaper than $650, at any rate.