I was advised to change mine every 15000 mile oldbiker and i have x4 NGK Iridium fitted in my twin pot 1800 ..They cost me £26 delivered ..Very happy with them .. A bit of a pig to fit but got there after a little cursing
.. I had 1 new one fail immediately on another bike ... Shit happens .I don't think there is anything cast in stone regarding any maintenance schedules Its all guidelines so some people have an idea of when it might be necessary to have a look as it where. I totally agree with you about shopping around for ALL bits as every penny counts these days but there is a point that cheap just doesn't cut the ice..and spark plugs is most definitely one of them... Oil is another
In My opinion to change something that does not need changing is a complete waste of money and time.