Some people have absolutely no respect. But that's OK. Karma will get them one day. Someone will eventually key their nice car and they will know how it feels.
Anyway, try taking it to your dealer (if they do repair work) or a reputable repair shop to see what they have to say. Maybe you could also have a professional detailer look at it to see if it can be buffed out with a machine. Although if it's right around the fin, it may need to be removed in order to work on the scratches.
As to how much, only a visual inspection will be able to determine what damage there is and therefore how much it will cost.
Good luck and I hope you get your back to normal once again!