Generally with optics you get what you pay for. NV for $600 is not going to be very good. For good NV your going to spend some money.
Your best bet and bang for your buck depending on what magnification you want is to get something from Trijicon.
Trijicon, Inc.
Their illuminated AccuPoint series is a great scope that competes very favorable against high end European scopes that cost 3-4 times as much. They also use their fiber optic technology to illuminate the scope in low light, ie early morning or dusk so you can take shots that you wouldn't be able to with a none illuminated scope. The 3-9x40 is going on my AR eventually.
Also don't forget to get a good mount, LaRue makes some of the best.