The supposed latest/greatest in the home-teeth-whitening arena are the Crest 2-Hour Whitestrips, which are currently available at Amazon for $32ish. But if you scroll down a bit on that page you should see an available $10 coupon, making a year’s supply of white strips just $22ish shipped. That’s under $6/treatment!
(Please note: Home whitening systems like these are great if you don’t have sensitive teeth. If you have sensitive teeth, stuff like this will increase the sensitivity. So be careful. But enjoy if you don’t need to be careful.)