Except for mason jars, I believe wood pallets are the most useful and upcycle material for DIY projects. And since they are made from a sturdy wood they are excellent for use in DIY outdoor projects such as building a deck. The folks at Remodelaholic.com have done just this using the pallets as their base for their awesome looking deck. Just check out the finished deck pictures and be azmazed.
Of course the cost will be low as you are using pallets for the base of the deck, but as with any DIY project you need to do the work and have the time to do so. A little know how helps to. But Remodelaholic does a good job of providing instructions for the newbie deck builder.