Far and away, the absolute best photographic bargain for under $100 is...a styrofoam coffee cup.
No, I'm dead serious.
It is better than any white balance tool you can buy, with the exception of a Spectralon target (which will get you back as much as a 7D).
Put it in the scene, shoot it, and you can eyedropper sample the light from any direction. You can even crop the image in ACR, adjust the white balance until you've got as uniform a peak as you can, and you've got a white balance that's the average of what's in the scene.
Or, put it over your lens and use it like an ExpoDisc, only better.
Why is the cup actually better than all the other targets? Because styrofoam actually has a nearly perfectly flat spectral reflectivity. The paints and plastics and what-not the expensive doo-dads are made from don't, because they need to build something that has a bit of durability to them. The styrofoam is also at just the right amount of reflectivity, in the 80% range -- bright enough to be noise-free, not so bright that you're in danger of clipping if you're careless.
To be avoided: paper and fabric. With the exception of some expensive "fine art" papers, all paper is pale yellow tree cellulose fibers soaked with fluorescent blue dyes to trick the eye into thinking it's whiter than white. It's the worst possible white balance target. And not only do most white fabrics have at least a bit of the same types of fluorescent dyes in them, they're translucent. Take a white balance off of somebody's shirt, and you're also white balancing off the skin, in addition to the fluorescent dyes in the detergent, in addition to whatever color remained in the fibers after being bleached.
P.S. If you want to do better than styrofoam, you can use a color profiling target (and the ColorChecker Passport is pretty good though far from ideal) along with some specialized software and a very funky workflow. It's actually guaranteed get you perfect white balance and exposure if you do it right, but it's very rarely worth it outside of carefully controlled color-critical environments...and, if you're in one of those, you hopefully already know what you're doing. b&