You have the Nikon 40mm Macro lens which is a DX lens, its a good lens and will do the job but if you want to shoot bugs then you will have to get really close to get the details you will want.
Nikon 105mm both D and G versions are good but I think above your budget.
Nikon 60mm 2.8G is the lens I would recommend used, I used to own it and it was very good, super sharp and while you still need to get closer to the bugs then the 105mm lens I got to shoot enough bugs to know its not bad for that.
Its also a great portrait lens on crop senr like your camera and a general use super sharp lens.
I sold mine around your budget price so I expect you can find it in the 300$ range, there is also a Nikon 60mm 2.8D and while I never used this lens I heard its excellent too.
another option is a third party lens which I am sure some people here can tell you more about then me.
Good luck