If it works for you, the REI UL60 (~3.5 lbs.) (it has a new name which I've forgotten) is relatively low-priced and often on sale.
The new ( and genuinely improved) version of the UL60 is the REI Cruise UL
. The material is tougher but the same weight. The whole package is 3 lbs, 2-4 oz (depending on which torso length you get). For the money, it's an awesome pack. When dividends come out in March, and this year's 20% coupon hits, you can grab one for $104.