Weell, you can get a LuxuryLite pack(http://www.luxurylite.com/stackpackindex.html) at around 2 pounds/56L that is external frame. At 2 pounds I don't consider it as a UL pack, which is why everyone is(or should be) here. The cost is fairly reasonable given Gossamer Gear and others have raised prices so much. Less than two pounds is what I would consider a UL pack. Around 1 pound is good. There is no reason a pack has to weigh more.
The biggest reason I have about 4 packs I use relativly regulary is the overall SIZE. Not so much for my base gear. This changes very little, a couple pounds one way or the other. Maybe another 2 pounds if I need to bring a bear ball. But the duration I will be out, with no resupply stops makes a huge difference in my pack choice. My activities change my pack pack size. Again, my base gear does not really change that much.
For 1-2 weeks I use a 22L pack (Murmur). For two-three weeks out I use a 50L Miniposa (now they call it a Gorilla.) For longer 4-6 week trips, I use an old 75L Trek. This sort-of depends on what I will be doing. If I am fishing, or photography, or climbing peaks, or canoeing, I may choose a pack size up or down.
Note that I chose all frameless packs. With a good pad, these all have good suspension systems. Yes, my back is always sweaty. Even at 35F my back gets sweaty. this is the trade off for using a frameless pack, besides being a pound to three pounds lighter.
Food is always my biggest weight and volume. Maybe not for a couple nights out, but my usual trips are at least a week or two. (Not this summer, though...a couple broken ribs has slowed me down.) On a through hike, the larger volume means I can resupply at a store, including a box of donuts, or a few pieces of pizza. I cannot buy just a half liter of fuel, I need to buy a liter. I cannot buy just one bandage, I need to buy the box. I use corn starch for crotch-rot, I cannot buy a baggies worth, I buy the box. I need duct tape, well, you guessed it, I buy a smaller roll, not just a few yards. On the trail, such resupplies never pack as compact as what I can manage at home. IFF you have to resupply, bring a slightly larger pack to allow this. I hate to resupply due to the inevitable volume increase...and extra weight. And dehydrated beer just does NOT taste the same.