-- Last Updated: Oct-23-12 12:37 PM EST --
You only have one head.
Bike helmets and many similar don't provide the cover for your head you need to care about in paddling. A good paddling helmet will protect forehead, down to the bottom of your skull in back and it is advisable to get one with ear cover as well. Those are all places that you can hit something hard like a rock when paddling that tend to be less protected in many other helmets. They are also places that something like a biking helmet doesn't usually cover because it isn't a likely whack point and they'd be unbearably hot.
You can go cheaper, and figure on replacing the helmet to be safe after one good whack on something hard, or go pricier and get something that might take more than one typical ding before being a risk. There is also the comfort factor - since you might be wearing the thing for a day without break, the more expensive ones often are made to be more customizable and comfortable.
I have been upside down and found a rock, in a small riff of moving water that was not at all the nastiest stuff I've been on. I can guarantee you that my bike helmet would not have left me unaffected like my paddling helmet did.