If you're buying the screens retail, probably $15-$20.
If you're buying them in Nintendo size volumes (figure a minimum 5 million for a worldwide launch) probably a couple bucks each.
Better resolution probably isn't going to be terribly meaningful when you're talking about a screen that small. Better processor might be possible, but I'm assuming the ones chosen were picked not only for power, but maturity of development environment (and high volume availability).
Honestly the CPUs are probably better than what the N64 offered (certainly easier to optimize for), so I don't think CPU cycles is going to be a limiting factor in too much. The only possible benefit would be uncompressing data on the fly. Frankly, with the evidently high capacity of the game media, that's probably not going to be terribly neccesary (the thing can probably do DivX decompression in real time for a screen as small as it is).