Welcome, OldTroop, to the site.
The cheapest route might be to build an AR. Kits are available which include everything except the stripped lower receiver. The lower receiver is the part you’ll need to buy through an FFL (well, it at least must be transferred to you through the FFL) or at a gun show.
Most of the kits I’ve seen come with completely assembled uppers. The only part you need to assemble in the lower receiver. You may choose to buy just the lower receiver parts kit, a stock kit and a complete upper, all separate of each other.
Building a rifle will allow you to buy things a piece at a time (maybe then you can avoid selling another toy to fund this project). It also allows you to customize everything.
Use good quality parts and you’ll save some money, customize the rifle and have fun doing it. I like the RRA or Eagle Arms lowers and RRA lower parts kits. The RRA and Bushmaster uppers are both great.