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.
So, mining should be extremely profitable? Why would anyone buy BTC for $1400 if he could mine one for $50?
It's obviously a lot more complicated than he is alluding to. He's also assuming that equipment that would get 1BTC/day in February would still be able to do it in February 2015 365 days later. That's not even remotely likely.
From what I can tell, the bulk of the harvest comes in the first few months and then as difficulty increases it slows substantially.
I don't think most people are mining coins at $50 a pop. I'm not sure that anyone is right now. That sounds incredibly low. I was expecting to hear something closer to $300