geofelt :
It costs 15% more, but you get 40% more performance because of overclocking.
To get 40% more performance out of an i5-6600k, you also need a high-end overclocking motherboard, a high-end cooling solution, some higher performance RAM, likely a better quality PSU and some luck-of-the-draw.
The build cost difference will be 30-40% higher for the CPU+MoBo+RAM+HSF combo, in large part due to the need for a high-end aftermarket cooler.