Metal would be ideal as it last longer and is stronger, obviously. I would look at craigslist and marketplace, I often find metal roofing on there, sometimes a bit rusty, but just on the surface.
Plastic can also work, I have a friend that uses plastic in the same manner as above. I think they have had it up for about 3 years and its still in good shape, other then some staining.
looking at home depot, if buying new then its about the same price for metal VS PVC roofing (which is what my friend uses), with the pvc being about $2 cheaper for the same size. Not sure of the size of setup you have but even if buying new you may be able to keep it under $100-$150 which isn't too bad.