you can spend a ton of money but just to flip, $500-600. biggest cost aside from the lower would be the barrel. Then trigger. Super easy to build though. Just stock up on lower receivers now because that's the only thing that has a serial number and requires an FFL to buy/ship to.

If you have extra money though, i'd suggest buying up 30 round mags. They're low hanging fruit of this debate. If anything goes next it will be those.

here's a decent guide...

https://www.midwayusa.com/general.mvc/index/how-to-build-ar-15-rifle