For the sake of offering an opposing view...

Wilko wrote:
-Decide on how much you want to spend on a helmet. (If you have a 20
dollar head, get a 20 dollar helmet. :-) )

This one is very popular to state, but I'm not convinced it has any value.
For many whitewater helmets, the high price goes to support nice paint and
hand-made, small production runs. That doesn't translate to added safety.

Motorcyclist Magazine had an interesting article on the subject recently.
http://www.motorcyclistonline.com/ge...helmet_review/
While the helmets they examined are not suitable for whitewater, some
of the same observations probably apply.

A helmet that is made of kevlar or some other composite material will
generally be stiffer, thereby spreading the force of the blow to your
head over a bigger area, and it will usually need (a lot) more force
before deforming. A plastic helmet is more likely to deform locally and
in that case it will pass the energy on the point of impact almost
directly to the skull below it.

Kevlar is a fiber with a high tensile strength and low stretch. That
doesn't mean it doesn't deform easily. The epoxy or plastic used is largely
responsible for the shape holding characteristics. As far as the suitability
of (pure) plastics or composites, the Motorcyclist article found that a
cheap (in price) plastic helmet outperformed (transferred less energy) the
composites they tested.