Generally speaking, the i5-4670k offers on average a 5% increase in performance over the i5-3570k. Based on prices over at Newegg.com, the i5-3570k is $220. From the perspective of the just performance increase (excluding overclocking and other new features in Haswell), a 5% increase in price to reflect that performance gain means that the i5-4670k should be selling for $231. However, the i5-4670k is selling for $240 after a $10 instant rebate. That means while the performance increased by 5%, the price increased by 9% or $11. Without the $10 instant rebate, the price increase is about 13.6% or $30.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116504
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116899
Of course, there are deals out there that can sweeten the deal for Haswell. For example, purchasing the following Asus Z87-PLUS for $180 give you 8GB (2 x 4GB) of free Crucial DDR3 1600MHz RAM ($70 value). However, not everyone wants that motherboard, and there are cheaper Z87 chipset motherboards as well. The cheapest one listed on Newegg is the MSI Z87-G41 for $109. Add in the cost of the Crucial RAM and you are back up to $180.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131980
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130695
Naturally, the Asus mobo is better than the MSI mobo and in the end if you are paying the same price, you might as well go with the Asus mobo combo. Of course, there are also less expensive 8GB DDR3 1600MHz RAM as well.
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If you are fortunate enough to live near a MicroCenter, then you can pickup an i5-4670k for $200 at the store. That's a savings of almost 9% or $20 vs the i5-3570k from Newegg. Of course, that is
before taxes. Since I know of no city off the top of head that charges more than 9% sales tax, at worse that means the i5-4670k from MicroCenter + tax will equal the price of the i5-3570k from Newegg.
http://www.microcenter.com/site/brands/intel-processor-bundles.aspx