The cost of mining varies GREATLY depending on what hardware you use, where your from and what electricity company you use. There is no average.
Anyways, here is what it roughly costs me (I have 700MHash):
Revenue per month: 193.69 USD
Power cost per month: 131.49 USD
Monthly profit: 62.20 USD
If you take into account all the time I spend fiddling with it and the cost of the hardware, I'd be making a loss. Nowadays due to increased network difficulty, mining is something you do for fun, and not solely to make a profit.
Here is a link to a handy calculator you can use to estimate your mining income, cost & profit:http://bitcoinx.com/profit/index.php
And a page that compares mining hardware:https://en.bitcoin.it/wiki/Mining_hardware_comparison