Since they come from factory with Platinum plugs, I would think that you still have ~20,000kms left (Nissan replace them at 80,000).
A few of us have done it.
I do not have the NGK factory plug number on me, but I believe that they are the same as an Evo 9 (at least the Denso Iridium's are the same between the two cars).
You can do it yourself so all the cost will be in the plugs, but you will need to take off the intake piping and removing the piping into the throttle body on the left side of the engine can make it much easier also.
They are pretty deep so a socket extension is required (at least it was for me).
As for plug cost, we had a small group buy earlier this year that you can read about HERE