If you already own most of the cutting tools and the drill, this instructable costs about $13-15. We got everything at Home Depot, but you can find similar stuff at any hardware store.
2 big paint buckets that stack (~5 gallons each)
1 plastic tub OR drain grate (The height of the tub/drain grate should be approximately the same height as the gap between the two buckets when stacked)
1 2' long 1" diameter plastic pipe (make sure it is longer than the height of the buckets when stacked)*
1 mesh baggie (find them as packaging for fruit, veggies, other stuff!)
drill with 1 inch bit and 1/4" masonry bit
utility knife with extra blades
tarp (collects all the plastic bits!)
black plastic garbage bags
seedlings or established plants
*I've read different things about using PVC after making this first version, which does use PVC. This project is made entirely of plastic, so if plastics in general bother you, you probably should not make this. If PVC specifically bothers you, it's easy to find other plastic pipes that will work, just poke around the garden supply store. Also, Greenpeace has a big database
of alternatives. What do you think about PVC? What alternatives have you discovered?