Whether or not you purchase a complete upper or build it yourself, the cost/expense is still going to come down to the quality and manufacturer.
The cost of the tools is negligible (if you need a torque wrench, AutoZone has a free loaner), and oftentimes there are other NES'ers that will loan you there's and/or assist you in assembling it.
One thing you might want to consider is if you plan on adding a muzzle brake or not.
I haven't shopped around in ages, but I'm guessing the number of vendors out there that off a wide range of AWB compliant uppers is limited.
Once the muzzle device is pinned/welded/silver soldered on there, you're stuck (unless you want to pay the cost of having it removed and a new one permanently attached), with the vendors offering of the sight base/gas block and rail system/free float tube.
If you really want to get it right the first time and to your liking, you might be better off with building it yourself.