copper ones are about $3-4 each, but only last about 5,000 - 10,000km, however they do give the best spark.
then you have iridium which are about $30-40, last 50,000km? and give a slightly weaker spark
and then there is platinum, no idea on price, but they last about 100,000km and give a weaker spark again.
i've only ever used copper, however i'm contemplating iridium as the rear 3 plugs on my legnum need the plenum removed (not as hard as it sounds though).