I think it would be an OMG moment if they released it at $400.
I know some people are anticipating a Sony price drop to offset the Scorpio, but that would be silly on Sony's part IMO. They already won the gen, and should really be focused on profit margins. I doubt they reduce the Pro price for the same reason they took forever to drop the PS4 price- they are selling a lot of consoles anyway and making a lot of money.
Back to Scorpio, it looks like an semi-equivalent PC rig could be could be had for $500. Ofcourse that doesn't account for all of the R&D MS put into this. But it also does account for the savings they'll get buying in bulk, not their (suspected) intentions to make this device forward compatible with xb2 software.
I don't think MS expects this to be a money maker, and is fine to let Xb1s continue to be their main seller. But I also think they'll be keen to garner the type of excitement Sony's been getting when they announce pricing .
I know it'll be between 400-500 bucks, but I have no idea where.