I agree, $400 is a lot to come by in one fist. I didn't see how I could do it not using any paycheck or credit. I also realized it would take $800-1000, $400 is a real tough goal.
I built mine. Bought parts as cash became available. I sold off knives, tools I don't need, refunded stuff I thought twice about, and scrimped by. My build has realistically cost over $850, almost done, financed entirely by selling off things I didn't need. It hasn't taken a dime out of my paycheck yet.
I bought an AGP stripped lower with trigger adjustment screw on sale, $79.80. You can get many like it for less from others. The A3 upper was a LAR factory blem, $48. I've bought a lot of parts from Brownell's with military discount, the kits bundled that way really cut down shipping. I wanted 6.8SPC, which isn't mil surp cheap, no $89 barrels from Sportsmans Friend there. The major maker is ARP, and the barrel shipped with headspaced bolt, cam pin, and firing pin, plus nitrided bolt carrier, was the most cost effective. It all came assembled complete, ready to install in the upper. An A1 stock, TD grip, A2 rifle handguards in Foliage Green, almost finished.
Definitely not another M4gery, not black, and certainly not like all the others. Arguably better than milspec. I might have saved some with 5.56, the difference could have been spent on a more common roll mark, or fancy tools.
I used a pair of vice grips, old drill bits as punches, a set of $8 vice jaw inserts, and 12" channelocks to tighten the barrel nut.
It's not rocket science.
Start with how much accuracy you want, then buy the barrel/bolt, upper, optic, furniture, and trigger in that order to match. If it's a plinker/SD/hunter, 2MOA is standard. That pretty much eliminates sniper stocks, adjustables, free floats, or 2 stage triggers, which cannot improve accuracy more than what the barrel shoots anyway. Sure save a lot of money, and more shooters would be better off doing it that way. Too many go at it backwards and wind up with $1200 2MOA shooters. Lots of pretty parts, no heart for the job.
All American parts, no known imports, 100% just repurposing what I had. Haven't had to sell off another gun to do it. It can be done.