The Nikon is pretty good if you'd prefer a fixed power scope.

I'd suggest the Leupold Crossbones which is a 2-7X variable that has 5 aim points built into the reticule which is designed for crossbow use. It's at the high end of your price range but is as good a crossbow scope as I think you'll find for that price range.

Really when it comes to made for crossbow scopes there's only one that beats the Leupold IMO & that is the Vortex Viper XBR scope. The Vortex has a much larger objective lens, is illuminated, 2.5-10X 44mm scope. Many advantages of the Vortex crossbow scope, no other made for crossbow scope I know of comes close to it...............BUT.........it's also 2X your prefered budget @ $600.......so that is a deal breaker for many.