Asrock X99m Extreme 4 - £120
This was my home server for a while. It runs well with retail and ES v3 & v4 Xeons.
Xeon E5-4610v3 - £50
Retail version. 10 cores, 20 threads, 1.7GHz, 105W TDP, no turbo. The high TDP and no turbo means it should run predictably at exactly 1.7GHz all the time...