Historically? Eh... I've seen anywhere from $0-$200/300 and I think the reason for the big discount (and I'm talking like 4-5 years ago) was because Apple was falling behind in the Gigahertz war with the G4. At one point they even put out dual G4's across the line with no single CPU model to help make up for the loss of ground they were experiencing.
I think, one time a while back, an update to the line actually cost more. Something like $100 more. Does anyone remember that?
~Crawn