Well I can see the theory of it being affordable. Even 400 bucks at launch wouldn't be too unreasonable. Even the Delux Wii U with only 32 GB of Memory is going for 350 bucks.If they decide on like 450 or 500... maybe not quite as appealing... but 400 bucks, not too far out of the realm of possibility.
As for backwards compatability, I'd suspect they might still use Bluray technology, since hell they've hardly ever had a game that tapped the 50 GB on it, so maybe it's still possible to get?
If not, I propose to Sony, since they would read this post... that those who buy a brand new PS4, get a voucher for either a specific amount of money, or a specific amount of games, that they can perchase on the PSN (limited time offer). They would be loosing money technically, but maybe it would encourage some people on the fence about getting the new system, since it would allow them to pick out a few choice games on the network while new games are in development.