If you want cheap why not buy Anderson as bare minimum. Rousch sports, KAK, aim surplus, matrix arm and damage industries are pretty cheap too. Picket Mill Armory also has some cheap but decent barrels. They also have discounts usually.
Anderson Sport Upper $40
Anderson Lower $40
KAK Buffer Tube Kit with stock $40
Anderson LPK $45
Matrix Arms Handguard $55-65
Aim Surplus Nitride gas block $20
Generic Gas Tube $5
KAK Over-run Barrel $110
KAK Milspec charging Handle $20
AIM/DSG arms Nitride BCG $90
A2 Flash Hider + Crush Washer $15
$490 right there for a carbine that also has a slim rail with everything made in the USA.
Rousch is pretty nice for budget builds. Also they make some interesting flash hiders, and they are nitrided too.
The trigger in the Anderson sucks tbh. I'd spend 20 dollars for for the PSA trigger. The trigger I got from PSA was polished and feels almost as good as the ALG QMS I picked up.
Here is a budget build I put together.
Foliage stuff/sights not included, I think I spent under $425 on this. Also the flash hider is from Rousch. Concussion doesn't seem as bad as the standard A2. You don't have to buy questionable parts to keep things cheap.
The rail is a cheap Chicom clone of the NSR,, but it's actually pretty decent. the keymod is in spec and the finish isn't bad. If I knew about matrix at the time, I would have got that instead. But it seems light and sturdy. I might just order a shorter matrix rail as a replacement. It cost the same sadly.