some cheap parts are really gonna be JustAsGood™ although they might not feel as nice (lower, lpk, upk, fcg, ch, muzzle device - the $5 a2 birdcage is still the best performer lol), others you should never compromise on (bcg, barrel). good inexpensive bcgs are toolcraft/microbest's mid-tier models, for barrels avoid rosco/ba.
if you're price sensitive ion recommend building the upper, you'll need a vise/tq wrench/armorer's wrench/reaction rod/specialty greases/etc. which you'll likely never use again. you can pick up an inexpensive psa which will come with a good fn barrel and high-tier toolcraft bcg, but the way they assemble them precludes you from ever doing any real customization.
nobody's buying shyt rn and dealers are desperate to move stock, you can legit put together three serviceable ar-15s with a $1k budget