N450 has integrated North-bridge and graphics, that's why it's pricier. Performance is about the same, but N450 systems are more power efficient. The graphics are much better (but not great) than the 945GC and not as good as Ion chipsets. Netbooks only chips, so you really don't see the per chip price in a system much.
If I got a really good deal on a N280 system, I wouldn't turn it down.