Here's a pic of some raised garden beds I built that I'm kind of proud of. Not for the craftsmanship at all but because I think it's a good, practical idea. They're really simple to build and the material is scrap cement siding that I found lying around. You won't find any expensive cedar wood that only lasts 10 years around here.
If you try to buy a similar raised garden bed at Home Depot or Lowe's, it will probably cost $80 or more from what I've seen.
Steps:
1) Scavenge 4 cement siding boards around a home construction site. You're looking for about 2'-3' in length, I would say.
2) Cut the boards to length to make either a square or a rectangle.
3) Find a 3/8" masonry drill bit, drill, and a circular saw.
4) Use the drill bit and the circular saw to cut a slot, halfway down the end of each cement board. So you now have 2 slots in each board, one on each end. I know this isn't explained very well -- just google "Scandinavian Saddle" and you will get the idea.
5) Just fit the 4 boards together like a log cabin and you are done.
I hope you can see the attached image.