Yes.
New AR-15s (M4gerys) can be had for ~$6-700. $350 will get you 1,000rd. to feed it. Used are even cheaper.
If you decide you want a full-auto variant, you'll have to fork over ~$15,000 for a pre-86 model. However, full-auto gives you little-to-no advantage over its semi-auto brother.
$500 for a an 'assault weapon' isn't really that unfeasible for most folks.
p.s. a $50 .22lr rifle can kill a person (with correct shot placement) just the same as a more expensive 'assault rifle'.