If you're not too particular about your parts, it's still cheaper to assemble your own AR than purchase a complete rifle. Reference the PSA links above, there are cheaper parts still, Primary Arms is still selling Anderson lowers for $48, granted they sell out quickly when they're in stock. JSE had Gadsden Flag engraved Anderson lowers for $68 this past week, again sold out in < 3 minutes, but set up in stock notifications at all the vendors and make sure you have push notifications set up on your phone's email, there's no reason you can't eventually get something even now.
I've acquired all the parts to finish off a pair of lowers, been shopping since November, one is a 7.62x39 and the other is a 6.5 Grendel, they're all good parts, only thing I kind of cheaped out on was the handguards and one of the muzzle breaks, I've got less the $600 in the 7.62x39 and less than $700 in the Grendel. I even snagged a blem Stag Arms lower for $63 back in December, so now I've got another to build...
There's a lot of cheap parts now, way more than when I got started years ago. Even with the market kind of turned on its ear, stuff is still available. My brother-in-law built a really cheap AR out of a bunch of parts from 22Modsforall and a PSA lower. I had my reservations, but it's accurate enough for what it is, easily "Minute of Deer" accurate at 100 yards.
On the subject of 80% lowers, understand that's very much a marketing term, percentage finished matters not to the BATF it either is or isn't a "firearm." Marketing calls it an 80% because generally that's as complete as it can be made and not be considered a firearm.
I was an early adopter, I've got one of BRS' first or second runs of Billet 80 lowers. It didn't come with anything, just a chunk of aluminum that looked a lot like an AR15 lower receiver. It didn't come with a jig or any tooling, I had to purchase the jig in addition, I already had some of the tooling, but had to buy a longer end mill. You've got to mill our the fire control group pocket, trigger hole, and drill the fire control group pin holes to finish it.
All said and done, it's a nice lower, but there are WAY cheaper ways to get a lower, as cheap as Anderson and PSA lowers are (were), unless you just want a project, or can't buy a finished lower where you are, it's not worth it. Even the cheaper "80%" options all seem to cost more than a finished receiver, as long as you're not a felon and you live in a free state, there's no economical reason to build an 80%.