Thanks for the replies. Looking to not go much over $600 for the upper. I think I can sell my 20" upper for 450ish and want to stay within $200 net for a lighter upper. I'll check out the rock river upper and see what they have on sale, and the PSA ones too.
If you want a lightweight upper get a lightweight barrel from any brand that fits your budget and use a magpul handguard instead of a rail. Those two items account for pretty much all of the actual and felt (swinging barrel) weight. Like a stabilizer, the farther away from the pivot point (your shoulder) the weight is the more you'll feel it.
I would personally recommend building one yourself if you have access to a torque wrench for the barrel nut. The rest of the upper can be assembled with a crescent wrench and something to drive the pins in (punch/channel locks). If you get a gas block that uses set screws or is clamped by socket head screws they usually provide the needed allen key. There's tons of how to videos online and it's a fun afternoon project!