Actually you can do 50k km with Iridium plugs... (see the link posted by @jaggernaut earlier). You'll obviously pay more but not change them for like 3 years.

I've done > 30k km with platinum plugs that were bought at a total of 5k back in 2010. Cars power output and fuel consumption still quite good. My car's engine utilizes only platinum plugs. Tempted to test that 50k barrier...

So back to 'rudimentary' math for @Impunity's sake:

30k km for 5k with the original plugs or 30k km for 9k with 'other' plugs. Keep in mind that the price has not been static during the 30k period.

There's also the issue of different plug design - plugs, gaps and what was considered for your car engine, with relation to engine power output and fuel efficiency. I know of someone's Subaru Forester (naturally aspirated) that was fitted with 'other' plugs that could hardly climb a hill - this is a 2L car with 137 bhp. Plugs were changed to the original and car was back to 'normal'.

Really depends on the engine. Some cars original plugs cost like 200-300 bob a piece, others cost 1500-1800 a piece (currently). The life time of each is different (obviously).

@Odec - probably origi - that was 2010 ... mine were 1200-1300 ish then. And if they are still good now, means they were platinum - assuming you don't do like 1km a day of driving.