My investments in new glass to supplement the 18-55mm kit lens that came with my D40 have been done on a low budget - about £66 so far, including postage costs.
One of the "limitations" of the D40 is the very small range of lenses it will autofocus with but, if you are not afraid of doing things manually, you have many more options. I picked a
Nikon E series 50mm f/1.8 and a
Raynox DCR-250 macro adapter (follow links to see the shots I have taken - you can also see
kit lens,
kit lens + Raynox and
50mm + Raynox).
Here is a shot that combines the two:
Wulf