depending how bad the scratch is, if it's just a scratch on the AR coating or just a small enough scratch but can be felt by your fingers, sometimes it can be repaired by urself.
google up diamond paste, i've done it before, works as a charm but it'll take away ur AR coat.
it's ez enough but to find the diamond paste is a bit tricky here in malaysia.
think about 100RM+ u will have both. comparing this method and going to the service centre to change the crystal, i think u shd try this first.