I just built the carbine on the top in my pic below for less than $570. It's a used Olympic Arms Plinker Plus upper ($200 shipped) with RRA bolt and carrier ($109 shipped), a cheap slipover/reverse flash hider ($15 shipped) to make it look more XM-177-ish, First Samco grips ($38 shipped for both), a Leapers bolt-on handguards rail (free from a 2-pack from an old build), a DPMS lower w/DPMS parts kit ($165 total), a used Bushmaster 4-pos stock from the EE ($40 shipped, and it came with a free Viking butt pad and Cherokee cheek riser too that I may sell to get some of the price back), and a homemade sling. It's the lightest AR I've ever held, and I've hugged lots of them at the gun shops.
Getting close to the $500 is easy if you don't want it customized. If you need vertical foregrips, rail systems, and namebrand stuff, you'll see an increase in price. Whatever you build, you're going to tinker with it in the future and spend more money on it anyway. Might as well try to get what you want now by building it your way the first time.
Keep in mind that you pay sales tax, fees, and all kinds of woopdeedoos on the ticket when you buy one from the gun shop too. That sticker price is where your spending starts. It's been cheaper for me in the past to order weapons online and have them shipped to the gun shop, where I pay a $20 transfer fee. That beats the heck out of sales tax and markup at the gun shop in almost all cases.