$400 max, and that may be too much. Sony is in a very precarious position with their next console.
In Japan, they are facing a WiiU that will have a year's headstart (unlike Wii), and has decent processing power (unlike Wii). Even if the PS4 is significantly better WiiU, if it is also significantly more expensive there is a very real risk of the system crashing and buring. Sony no longer has exclusive 3rd party support to depend on. For reference of possible outcomes, see 3DS and Vita.
In the US the situation is similar. WiiU will likely take a big chunk of the market and then there is XBox360. Speaking as a consumer who has low loyalty to systems, I can say that after being happy with my PS2 I bought a 360 over a PS3 almost entirely because it was significantly cheaper. I think I represent the mass market pretty well.
For Sony to avoid losing more market share it absolutely must offer a compelling system at a good price. I think even $50 over WiiU may spell failure, but that's the limit and it would have to offer something really good.
