You can put together a pretty good AR-15 from parts for right around $500 if you shop around. Very good will run you about 600 to 700. Figure another $50ish for the special tools.
Far as putting it together borrow the tools or buy (resell em after) if you plan on making more. I think headspacing and barrel install was the most technical part and that’s not too bad either, other than that anyone that can use basic hand tools and follow instructions can put together an AR-15 from parts.
Barrel install is definitely the most difficult part in my experience. I've also found that barrels come correctly headspaced - at least the ones I've bought.