As with any product, it's diminishing returns. I believe all factory AR should include, at minimum:
- Reputable manufacturer with good QA and warranty.
- No plastic major parts.
- HPT and MPI BCG.
- Property staked gas key and castle nut.
- Threaded barrel with modern profile and twist.
- Nitrided barrel and bolt.
- Free-floated full length M-LOK handguard.
- Better than A2 trigger and furniture.
- Quality magazine.
- Made in USA.
Imho these points are
essential but also make for an optimal launching point: all the core features an AR 'needs' -- but with room for cheap/easy upgrades. Further, upgrading barrel/handguard/etc. will cost $$$ plus tools and know-how. (e.g. In the end, you're likely to save money by covering these baselines out of the box.)
In 2022, for a used factory AR that checks the boxes, you'll need ~$800.