Well that sounds about right. That's the same sort of 4-year gap that I tend to stick to. The good thing about the 13" MBP is that they're basically required to keep the price lower. It's funny, though. We're in a point in time where for 90% of uses, the C2D CPUs from older Macs (or any PC for that matter) are fine. So that 2008 MBP would be fine for a non-power-user. On the other hand, the amount of people I've seen trying to keep a 10-year old PC running is astounding.
We have finite download limits here, so I much prefer to keep content locally than stream it. Audio's quite negligible, but still.