I was wondering about the weights of different configurations of lowers and could not find anything on the web, but I might not have been using the best search terms.
So, after I had all of the three most common setups, I weighed them myself. This was done with a small kitchen scale, so consider it approximate. A number of weighs were conducted for each lower and the weights below reflect the average. Therefore, YMMV!!!
Hopefully, this will be useful to someone. 
GOOD INFO!
I think it would be neat to weigh one of those 1-piece polymer lowers. Does anyone know the weight on them? Someone was building an uber-lightweight rifle at some point IIRC. I think it would be neat to build:
carbon-fiber wrap 14.7" barrel (if this is not procurable, pencil-barrel w/Noveske low-pro block)
Cav arms polymer lower
Aimpoint T1
carbon-fiber HG with rail bolted on and KAC grip and X300 weapon-light installed.
I think this would be a VERY lightweight all-around do anything gun. Why? I dunno, no reason really, just light and handy and well-rounded.