The copper ones are NGK and should be within 1500rs.
The ik20s cost between 4000 and 4500.
The ik20s last longer so they are actually cheaper if u factor in the life.
If you're looking purely for performance, I would suggest copper.
If you're looking for something u can put in and forget, then the ik20. No worries for 15,000-25,000kms. Apparently people have run them for 80,000kms as well, but I'm afraid such sad stuff won't be happening to my car.
Copper I change every 5k-10k.
Sent from my GT-I9500