If you have some spare cash, I doubt they'd charge much, MIJ would probably be able to make you up the pipe you need, and weld it to your current back box.
In terms of pricing I have no idea - I paid £650 for the entire system inc 2 de-cats.
Cheers i'll give them a call. I've just opened a complaint against the seller on Ebay as well, they really shouldn't be able to sell these systems as a direct replacement when they dont fit.
I will keep you posted how I get on.
Smokey