Saw an add on Craigslist for a used C14 K&N air filter for $55, so I checked the web to see what they go for. Regular price is $69.95, but found them on sale at amazon.com right now for $44.81 with free shipping (if you select the "Free Super Saver Shipping" option).
It says they only have 10 left in stock (but more on the way), for what it's worth.
The best price I found for a genuine Kawasaki filter was $34.28 (w/ free shipping) at Riders Choice Racing, so for $10 more you get a reusable air filter? Seems like a no brainer to me.