In my experience, the most cost-effective CPU in terms of ROI for mining is probably an old Intel XEON CPU offering like the Intel XEON E5 2680v4 featured ... lack of support for older CPUs ...

Our 2666MHz kit combined with the top CPU in the Ivy Bridge family will allow unparalleled performance." Kingston achieved Intel XMP certification for its family of HyperX memory specifically ...

This is taken from our review of the Dual Xeon E5 2687W on Asus Z9 PE-D8 WS running with 64GB of Kingston 2,133mhz Predator ...