they are considerably more durable than the resister plugs that are stock. i have used a set for nearly 25k and changed them to get the proper heat range for my riding style. when i pulled them they were still in good shape.
as far as lightning your wallet well at the initial purchase yeah some but when you factor in the overall cost your saving $$. due to not having to replace plugs as often and the associated time that goes with it.
you will NOT see a performance gain over stock you will feel an increase on the immediate if your old plugs are severely worn but remember it's just a restoration not an increase over stock.
Iridium plugs don't provide any performance enhancement, if that's what you are looking for...
They are more expensive over Platinum and standard Copper Cores.
However, the last longer than both.
Standard copper cores, you can expect to pay about ~$2/each and get about 20-25k miles out of them.
Platinums will cost you about generally run you about double of a copper core
and will last about 50-60k miles.
I don't believe NGK makes a Platinum plug that is equivalent to the copper cores.
Iridium plugs will cost you approximately $10-12 each and will last up to 100k miles (providing engine is in good running condition).
If you go with Iridiums, look to pay somewhere in the neighborhood
of $40-50 for a set.
That set will likely last you the majority of the life of the bike.
It's all relative. Per mile, they are about the same and will cost you
about the same over the long haul.
The only choice to make is to decide if you want to spend the extra
cost up front and not have to replace them as often or spend a couple
bucks every couples every couple of years to replace them.
I dont believe iridium are worth the money. I believe regular plugs last long enough although I pull my plugs every year just to verify condition, gap, color, etc.. Also don't want them seizing up in there, put a bit of threadlube on them before reinstalling.