As previously mentioned, it's about £35 but since applying I haven't had any noise at all. A couple of months with a weekly wash? I also supplied some to ybt200 over in Malta because he couldn't get hold of it. He reports good results too. The trick is to apply it to the felt strips. Boyd1963 mentions the GummiPfledge stuff which I'm sure does the same job. I couldn't be arsed to go the dealer route and book it in when I was fairly sure I'd be able to sort the problem myself.
Waxing I'm sure is the 'cause' of it. Or I should say particular types of wax/sealant in conjunction with the design of the door/window seals. The noise on mine only came on after trying a wax polish. Prior and since I've only used acrylic sealants.
And if any dealer tells you they're unaware then they're talking nonsense. When I enquired about the BMW/DuPont Krytox stuff the parts guy said it was for squeaky doors and that they keep it in for workshop use.