Probably you need new brake pads, and possibly a new rotor. But if you're lucky, you just got a pebble lodged in there and you need to get it out. To find out, take the pads off and see how thick they are. If they're thin, it's time to change them. Don't let it go, because the rotor can be expensive.
I don't know how much the dealer will charge you to do the job, but it's not very hard. I'll bet you could do it yourself if you can follow instructions and take your time to do the job right. A service manual will walk you through it step by step. The rear pads for mine are $32 from
http://www.ronayers.com . You could also try bikebandit or our sponsor (Beartooth Kawasaki) or your local dealer or lots of other places.
Curt