$400 for 1K rounds is pretty common these days, for cheap non-steel cased ammunition. Have you thought about reloading? You can get set up and reload for far cheaper in the long run. You can load FMJ ammunition for as low as $170 per 1K rounds.
You can get a Lee Turret Press Kit and .223 Dies from Graf & Sons for less than $250 delivered. Powder Valley will have all your brass and bullet needs as well as powders and primers. However unless you are going to buy in serious bulk quantities powder and primers are best purchased locally to save Hazmat fees.
Just google reloading calculators and start plugging cost of components into it. You'll see how cheaply you can manufacture your own ammunition. If you aren't going to shoot that 1K up very fast then maybe reloading isn't for you. However if you shoot your AR like my Wife, 1K rounds might last three range sessions. If you want to shoot more rounds for your money reloading is the best option available.