I can't imagine it'd be that much to replace the rear window weather stripping in a 2-door model. Installation is another story, but that's what the Bentley's manual is for =)
(Bentley's manual is the how-to for taking apart and replacing parts in your car)
Perhaps our sponsor has that part. What year and model is your bimmer?
Welcome aboard, btw =)
Oh yeah, and as for the seat, mine's worn through to the foam, and it's even worn into that quite a bit. I keep getting foam bits around the floor mats from where the seatbelt rubs the seat. Grrr.
This is a common problem for most of the bimmers I've seen. I'm sure some of the peeps here have ideas for maintaining and restoring leather. Mine, however, are completely shot =(