Believe it or not, NOTHING is not cheap. The lighter you go the more you spend. There are a number of ways to lighten an AR. The AR that we built for the PM convention raffle weighed in at 7lbs empty. Much past that and you better have your wallet ready.
The best ways to lighten the barrel is to go with a barrel like proof research, but you're looking at well over $500 just for the barrel. You could also find a good quality fluted barrel. Not a big gain in weight savings but, done properly, you don't sacrifice accuracy like you can for some pencil profile barrels. Faxon firearms has some nice fluted and pencil profiles, so does Wilson and Shilen.
Be careful with hand guards. A lot of the weight in those has to do with the barrel nut.
On the lower, you could go with a minimalist style of stock, such as a foot that mounts directly to the buffer tube, skeletonized lower, grip, or even a polymer lower.