On the PCT forums, a hiker tested footprints against cheap hardware store tarps. He said the 3 mil plastic sheeting didn't rip even in crazy wind last season. Saved a ton of money and worked just as well for moisture barrier too.
@hall.alex, the bit about 3 mil plastic not ripping in wind is totally true. I used a cheap hardware sheet under my tent on a long hike last year. It handled rough weather and wet ground just as well as expensive stuff.