If you are referring to a drop-in filter, they are "worth the money" if you keep the vehicle for, say, 100,000 miles. The fact that the K&N is re-usable means you don't have to keep buying paper disposable filters ever 10 or 20k. Don't expect a major (or even minor) performance increase, tho. I did a K&N drop-in on my '97 F-150 and the difference was very minimal (if at all). You need to do a CAI setup for a noticeable difference. Stock intakes are restrictive by nature (mostly to produce a generic torque curve and to reduce intake noise), and changing the filter element alone will accomplish little.
Try this: take your stock filter out and go for a rip around the block. Feel any difference?
Didn't think so.
But having said all that, I'm shopping for a deal on a K&N filter for the wife's ride...(Which we plan to keep for a long time!)