Think of buying an AR like buying a used BMW M3. Say the average cost for a used 2006 E46 M3 ZCP is $29,000. You are looking, and you see one that is going for $21,000. Through general common sense, you know that there has got to be something wrong with that car. Damaged, a salvage title, bad transmission, needs a new engine, or something similar.
The same ideology needs to be applied to a new AR. If you have one AR that is significantly less expensive than the average cost of quality market brands, you should surmise that you will be losing something by dropping your price. There are some great rifles out there for under $1000, but in the $600 range, you will get a lot of junk. You get what you pay for. Buy once, cry once. It would be much more advisable for you to save up $200-$300 and get a better rifle, like a Spikes Tactical or a S&W M&P15. Don't cut corners just to save a buck, because you will pay for it later. If you want a BMW, you don't cheap out and buy a VW and expect to get the same thing. Likewise, you don't buy a DPMS and expect to get the same thing as a Colt or BCM. I don't consider the DPMS to be a budget rifle, I consider it to be a CHEAP rifle (build quality and cost). If you want a budget rifle- one that is less expensive but still good quality - look at getting a S&W M&P15 Sport. The Sport took a more expensive rifle of good quality and lowered the cost by removing some features. They didn't lower the quality, just eliminated some options. It is the only rifle I would recommend for the price range you are looking at, as it's the only one in that price range that is not a steaming pile. It's actually a good rifle for what it is.