Hard to judge
I bought a damaged CSL bumper off eBay for £550 but that included 2 new BMW splitters and a new grille

and the damage wasn't half as bad
I was told by Simpsons that a repair like that done properly costs £400-500 plus spray.
I'd say you could get £300-500 for the damaged bumper if no splitters or grille were included.
Just don't do a cheap repair!