This bed (not including mattress) cost me about $40 including:
Bookshelf - Free
Piece of plywood (1/4" thick, the same width and length of the bookshelf) - $30, you could use any flat board like thing that does not bend easily
Screws (8) - approx. $1
Contact Paper - $10. You can keep it simple with clear contact paper or get more exciting with animal prints!
Screwdriver - already had
Drill - already had
Although you may not be able to find a bookshelf for free, check out yard sales and craigslist. You will be surprised how cheap you can get them for. They are often much cheaper than buying a used bed frame. Also, make sure the edges of the bookshelf are thick like this. If they are thin the bed will be flimsy and may not support your weight and it will be hard to attach the plywood.