I tried to build a cheap telephoto lens, I managed in it although mine has some limits, there is a reason why commercial lens are really expansive...
Step 1: Material
Material I used:
-Reflex body cap (for my Nikon D40 the BF-1A) 5 euro
-Magnification lens (diameter 10cm, focus about 280mm) 5 euro
-PVC pipe reduction D=10cm to 5cm 2 euro
-PVC pipe D=10cm 1 euro
-PVC maintenance joint for 10cm pipes 9 euro
-Self adhesive black velvet (and optional black rugged self-adhesive)
I also used a saw, a drill and a threader and screwdriver.