lets say the 7XXX series cards come out, and they do have 2* the stream processor count. This should mean they are more expensive than the previous generation (because of these improvements).
Yeah, because this is what happened in the past.
Dude, the WHOLE idea of the 7xxx series for ATI is to offer more power for the same cost. Due to the die shrink the plan is pack more than double the stream processors, at a higher frequency rate on top, into the same silicon.
No, it wont be twice as expensive, and it will an immediate have an impact on the old hardware prices when it is available. They will be a little more expensive, but not massively so.