I built my upper receiver on an Aero Precision stripped upper. With forward assist and a custom dust cover, it totaled around 120-140 bucks. I already had a bolt and BCG, so I am not sure without looking what that will run you now.
My barrel is a 10.3'' Daniel Defense Govt. profile. Around 250 bucks. Depending on what length you want, barrels can be cheaper or more expensive. IMO, CMMG's WASP treated barrels are nice and reasonably priced.
Flash suppressor is a Noveske KX3. Around 120 bucks. Tons of options here. Choose what you want. Price ranges from 15 bucks to 200+.
All in all, you will be looking to spend around, for a quality built upper, around 500 bucks, give or take.