I often see these listed on ebay for £400, but then again who has had CATS fail?
I keep mine in a box in the garage ready for when I revert to standard to trade/sell the car. You can then sell the aftermarket downpipes for a reasonable ammount.
Equally important to have as a backup if you fall foul of the more stringent MOT rules.
My advice would be to keep hold of em - £100 is nothing!
Car: 2010 Jaguar XFR (Cat-back exhaust)
Gone but not forgotten: Highly Modified E90 335i LCI M Sport Manual (circa 480BHP/500lb ft)