Well DS Arms is selling complete lowers for $180. M4 nitride uppers for $275. Charging handle for $15. Pick up a BCG for 105-159.
lets see that adds up to $ 575-629. Thats pretty inexpensive. Then there is the CMMG Bargin bin rifle....... So , Yes you can still get a decent AR15 for less then 600.
other options
M&A 20" 1 in 7 chrome molly barrel A3 upper(with M4 ramps) $335 does not include BCG or CH
DS Arms 16" nitride midlength without BCG and CH $300
DS Arms stripped blem lower $60
DD 1:7 cl 4150 button rifled barrel $169
Henderson Defense stripped upper $59
LAR Stripped blem A3 upper $45
DPMS upper parts kit $35( dust cover, foward assit and CH)
A2 flash hider $7
crush washer $2
gas tube w/pin $13
front gas block w/top rail $30
F marked FSB (set screw with sight parts $35
M4 6 pos buttstock assmebly $45
LPK $50-70