I have a Forester 2006.

Some bright spark has smashed my wing mirror (driver side). The whole thing has come off and housing is smashed. So just the wire is hanging out.

Anyone got a general idea how much these would cost to replace.

And are they model specific ? so for example if I went to Halfords , could they replace it with a generic one rather than a specific Subaru one ?

certainly won't get one in halfrauds lol
Best bet is looking/searching on eBay for forester SG models being broken for spares or used mirror assembly as direct search .
Used cost about 50 roughly . for new price you best sit down first :-)