Its not a rip off, its the fact that really you shouldn't be offically be allowed to order the other bits on their own
but a tip is if you buy these bits, then decide to buy the full kit, you can sell the new bits that are left from the kit pretty much for what you paid for them, well that is if you get a good price in the first place
Check out the UK group buy threads on the Exhaust and the Airfilter to give an idea of what you should be paying