The NGK Iridiums work out at 12.50 + VAT per piece. About 60 euro for a set of 4.
Personally, I change out platinum type spark PFR*B plugs for the Iridium type because they do not hold spark well at the higher rpm, higher boost or when you go for more power.
Case in point, the Time attack subaru that we mapped with water methanol injection needed plugs changed from PFR (platinum type) plugs to NGK Iridium no 8.