http://www.logicalincrements.com/
900 is pretty much the sweet spot, if you want to build for less you will get (more-less) for your moneys worth.
these guys are saying spend 1.5x on your GPU, 1x on your cpu, 0.8 on your mobo, $50 on your ram, $75 on your psu and $75 on your case. so if you only have $500 you are looking at around $135 on card (rx460) 90 on your chip ( g4600) and $70 on your mobo. And this doesnt account for needing HDD, which would only steal further from your PC and IMO dont even bother spending any money on a $500 full build as it will already be on its way out