Hi
It sounds as though you car has the NGK Platinum plugs in, I belive these last around 60-70K, but I have my dout about that.
If you can PM your engine and vin number I can confirm this, but if you have been quotred ?15.00ish each, thats sounds like platinum.
I can do NGK platinum plugs at ?8.09 each plus vat delivered to the UK mainland.
My contact details are shown below.
Stuart