I just got a Lowe Alpine Zepton 50 from Steepandcheap for $60.

2.5 lbs, aluminum stay, 50 liters, pack body uses dyneema fabric. I'd recommend looking around for one online, it's pretty awesome. If your back is the right size for it.

If you really, really, really aren't going over 20 lbs max weight, though, you should be able to get a sub 2 lb pack that does what you need. Probably sub 1.5 lbs. You don't really need a metallic frame at those weights, plastic and foam can give good weight transfer.