It is quite difficult to buy a sub-600 gram pack (@55 litre) ‘off the shelf’ for reasonable money. I have tried a number over the years and would not carry anything else. The ‘list’ below is not exhaustive but intended as a good starting point:
Gossamer Gear’s (now 54 litre http://gossamergear.com/g4-ultralight-backpack-all-bundle.html) ‘tried and true’ G4 lead the field (576 grams inc hip belt) US$150 and was my first ‘ultralight’ pack; (You can sew in some pieces of webbing to enclose carbon fibre arrow shafts for load transfer or, I find Big Agnes’ ‘Cyclone Chair’ (http://www.theultralighthiker.com/cyclone-chair/) underneath the pad works well) especially with the Airbeam pad as featured here: http://www.theultralighthiker.com/klymit-air-beam-inflatable-pack-frame-update/.
You could also try Gossamer Gear’s Gorilla which comes down close to 600 grams if you switch out the waist belt as I did: