If you want to use NGK 9s, they USED to cost £70 delivered from most places, although im not sure on the current price as a new version has just been released or something. Something to bear in mind though is that you need a different version of the 9s than what the cossies use (assuming you are using a coil pack on your engine). The cossies use R7234-9 and you need R7235-9. Using the wrong version will result in a small misfire issue as Stu @ MD can confirm ( i hope )

They should last quite a long time - they are the most highly reccomended and used plug (i would imagine) for the bigger powered cossies (ie 330+bhp)!!