I built one for about $350 - shoots great. There are plenty of videos online for how to assemble them and you’ll get the satisfaction of owning a gun that you made. I’ve made 3 in total, and it’s a lot of fun.
You just need to buy an armorer’s wrench for tightening the castle nut - about 20 bucks and the rest of the tools you probably already have in your toolbox.
I bought a stripped lower from a gun show for $45. All the rest of the parts you can order online shipped to your door. I recommend Palmetto State Armory, but there are lots of e-tailers with great prices.
This video may appeal to some frugal gunners.
Paracord AR-15 Trigger Job
https://youtu.be/Yeasg5HNbnw