<$500 would be pushing it. if you wait till $50 Palmentto lowers are available again and get one of their build kits you would be close but still over.
16'' M4 CMV, Chrome-lined, Rifle Kit - Rifle Kits - AR-15
you would be around $550 with transfer of the lower and shipping. you would still need mags, ammo, and a rear sight.
or would I be better off with a mini14 conversion project? I want a semi auto hi-cap 223/556 for the lowest cost possible that has the ability to hit a target 300 yards+ , I am not fixed on any particular brand so I am open to many options. all opinions appreciated