Simply said, it's the price of the mining gear and the cost of electricity.
The speed of mining 1BTC depends on your hash rate compared to the total hash rate, and of course the difficulty.
For example:
current difficulty: 908,350,862
nowadays you can rent hashing power, should include appr. the price of device + electricity: 10 dollars/GH/year
hash rate: 1825GHz
1BTC/day

So that's 18,250 dollars / 365 = 50 dollars/BTC

edit: 10dollars/Ghash/year is a pre order product expected in February 2014, so with the difficulty changing fast calculations won't hold for long.