you don't say front or rear.
the fronts are easier as it's a bolt on affair, but the rears will have to done by a bodyshop.
a genuine wing from VW will set you back ?70 inc VAT. I'd advise strongly to get a genuine one as I've heard horror stories of ill fitting ones that looks totally sh!te.
I had a vent put in my genuine wing at the same time after I discovered the same rust as you. It'll set you back anything between ?200-?300 depending on how good your 'shop is, etc. Just for a rough guide.
hope that helps.
oh, venom sell the vents. about ?35 after VAT and P&P. 2 styles availible. Mine's the T4 style. The other is the sprinter. 3 slits.