It has to do with how efficient these CPUs are at completing their tasks. The i7 970 uses an older architecture that cannot get as much work done per clock cycle compared to the newer i7 2600k, even with the 2 extra cores. The i7 970 is priced higher due to being on a more enthusiast oriented platform, LGA 1366, where a much higher price premium is expected, and it is one of the few hexacore CPUs available from intel. The other two hexacores are Extreme Edition CPUs priced around $1000. The i7 970 was a very powerful CPU when it came out around 3 years ago, and its price really hasn't dropped that much, even after better CPUs have come out.
As far as value goes, there is no reason to get the i7 970, the 2600k is faster in every way, and comes on a cheaper, and more modern platform, LGA 1155. LGA 1155 has a further upgrade path to Ivy Bridge, LGA 1366 is a dead socket, it's getting replaced by LGA 2011 in 2012. The only advantage 1366 has over 1155 right now is higher memory bandwidth with the triple channel memory configuration, and very few applications will take advantage of that. 1366 also has more PCI Express lanes than 1155, but that is only really relevant if you have a really high end triple GPU or quad GPU setup.