It is worth noting that lots of miners actually undervolt their GPUs, so they use less power, as opposed to the gaming community who overvolt to get the max speed out of them.

The logic is as follows:

V=IR and P=IV=(I^2)R.
R is constant so I is proportional to V. i.e. I rises by the same percentage as V, when you over or under volt.

Increasing the voltage by 20% may increase your hash rate by 10%, but it will increase your power costs by 44%. (1.2 x 1.2 = 1.44)
Decreasing the voltage by 10% may not affect your hash rate, but will reduce your power costs by 19%. (0.9 x 0.9 = 0.81)

So, the 10% increase in hashing by overvolting costs 72% more in electricity than undervolting for the original hash rate.