...except that when you think you're asking it to do just one "task", it's really doing dozens, each involving several thousand "things"... ;-)
I'll have to take a more detailed look when I get the time (as I mentioned earlier, I'm no longer a hands-on expert in the notebook arena). Ultimately I'd expect notable differences at a fundamental level with a price point difference in the 40%+ range, because (a) the overwhelming majority of the electronics in any of these boxes is more or less single-sourced and (b) computer manufacturers operate on notoriously thin margins, all things considered.
Generally, the bulk of what you're paying for in a machine (put the display aside for a moment) is the microprocessor & supporting chipsets. Desktop micros run (retail) from about $95 [CeleronD 2.8GHz 533MHz FSB 256K L2 cache] to $660 [P4 3.8GHz 800Mhz FSB 2M L2 cache] (there's another on the higher end at about $1100

and a couple dozen on the lower end - older socket styles). Notebook micros run $99 [CeleronM 1.4GHz 400MHz FSB 1MB L2 cache] to $468 [Pentium M 2.1 GHz 533 FSB 2MB L2 cache]. Contrast: $461 buys a desktop micro P4 3.66GHz 800MHz FSB 2MB L2 cache.
All the technobabble numbers (+disk drive type/speed + memory type/speed) translate more or less to how fast the machine responds when you click the idiot rodent. I'm kinda picky in that department, since I use the machines mostly for work - the fewer of my finite number of heartbeats I spend doing work, the better.