Sorry to hear this, hate it when I hear things like this. Happened to my 370 last year, a scratch about 10 inches Long along the back wing over the wheel arch.
Regarding cost it all depends how good a job you want. I went to a recommended place nearby who I trust, they did a perfect job across 2 panels and no one can see the old mark, it's spot on, but you get what you pay for, in my case it was approx £350 but the damage sounds worse in your case
If you are thinking of trading it in a garage will barter you down on price if you don't fix it and it will definitely put potential buyers off given there are mint zeds out there, so I would get it done. If you are trading in id look to pay less and get an ok job done but if keeping I'd spend more.