Forget about the "four figures" thing. There is no reason to pay that kind of money for a post-ban AR15. I can buy good quality AR15s from my local dealer for far less than "four figures".
If you decide to put one together I have some advice. Buy a forged lower receiver. This should cost less than $100 out the door. I have purchased several for around $70 plus the transfer. Second, as Harry Tuttle says, buy a good quality lower receiver parts kit from a reputable company. I use Bushmaster. The difference between a name brand parts kit and one you buy at a gunshow where you have no idea of their origin is maybe $10. Even if it is $20 I would rather have the piece of mind and also have some recourse if something doesn't work. If you buy from a place like Bushmaster you at least have someone to call if it doesn't work out. Lastly, buy a complete upper receiver with the bolt, carrier, and charging handle. As has been mentioned, you can get these used from a number or sources. IMO you need to buy one with a 5.56mm chamber and not one chambered for .223. You need to buy one with a chromed bore and chamber. Again, Bushmaster is the ticket.
When you get done, you will have paid far less than "four figures" and you will have a top quality AR15 that you didn't take short-cuts on. If you are buying a gun, you need to buy a quality gun. In this case, there is no reason not to.
The nice thing about putting a rifle together like this is that you don't have to come up with the full amount of money right now, out of pocket. You can have patience and keep your eyes open. As good deals come up, you can buy the parts. Over a couple months you will have a fine rifle. The other option is to jump right in, buy the cheapest thing you can find and possibly later regret it.
