O2 exhaust sensor WTF

[slo2]

Wifes bugeye STi just been in to get Engine management light checked out. (Came on during the week but car running fine ) Apparently an exhaust sensor needs replaced causeing the EMO light to come on. Only £169 plus VAT or £256 fitted !!!!!!!!!! Did not get it fixed. Felt it was alittle expensive!!! Has anyone else had this problem and what was the best solution? This is the very first thing to go wrong with any Subby that I have ever owned except for the bits that wearout.The car has a PPP kit bfitted complete with the prodrive exhaust although the dealer wanted to know if it had been modded? would have thought his computer thingy would have picked this up also the exhaust does have prodrive written on it and they have serviced the car for the past 3 years!!!