Firstly, cock to clock frequency comparisons don't tell the whole story as Haswell is a micro architecture update over Ivy Bridge and therefore provides up to twice the FLOPS/clock using AVX2, updated AES-NI encryption, better virtualisation support, etc.
But, using Scan...
E5-2650 v3 (£865) costs 2% less than an E5-2650 v2 (£882) and although the base clock speed is lower (different micro-architecture) it supports 25% more threads (10 vs 8)
E5-2660 v3 (£1070) costs 2% more than an E5-2660 v2 (£1050) but has 18% higher core clock frequency (difficult to draw a direct comparison due to micro-architecure change, but it's only going to be better...)
E5-2670 v3 (£1178) costs 1.5% more than an E5-2670 v2 (£1161) but supports 20% more threads (12 vs 10)
I'd argue that the price/performance is not the same.