Stinger or Mega fully assembled lower for $199 + JT distributing upper for $289
= AR-15 for $488+shipping and FFl transfer costs.
I think "Iblis" is offereing the Stinger and Mega lowers on the Equipment exchange. I was just checking out his website last night (he is a Stinger and Mega distributor) but I can't remember the address right now.
If time is not an issue, you can almost certainly get an upper for even cheaper on ebay- if you are patient you can usually find them in the $250 range for used ones. Might be a little harder to get exactly what you want, but if the $$$ is REALLY tight you could do it...
Granted, in this config you will not have a chrome lined barrel, but I assume that is not an issue if you want to spend only $500!
Don't forget you will still need magazines, which will run $15-20 a pop depending on brand and make...