The electroplated nickel on the copper block is so thin that it can virtually be ignored when considering thermal conductivity.
It would be stupid to make a block out of solid nickel. Its thermal performance would be terrible.
Silver is only 7% better than copper but is much more expensive. Pure silver also oxidizes quicker than copper. About 10 years ago, when silver was much cheaper, there was a heatsink, the Noise Control Silverado, that used a 50 gram slug of pure silver.
^Yes, it's pretty much an atomically altered derivative of graphite. However, given the fact that it's typically produced in a lab at the atomic level via sophisticated and controlled methods, I'm guessing that alone would put it financially beyond mainstream PC applications.
A pretty interesting read; and considering it was the basis of a Scientific Nobel Prize in the last few years, I think development into other areas of application will be explored before it makes it into a tube of TIM.
In 2008 graphene produced by exfoliation was one of the most expensive materials on Earth, with a sample that can be placed at the cross section of a human hair costing more than $1,000 as of April 2008 (about $100,000,000/cm^2)
They are working on methods of mass production that hopefully will allow for applications to begin to become practical in the near future. IIRC, in that same article it is mentioned that the price of a graphene coated silicon carbide wafer was $100 per cm^2, still a little pricey, but for a high end chip not horrible.