The money comes from people buying bitcoins. And people buy bitcoins to pay for things.
It's like Disney Dollars (
http://en.wikipedia.org/wiki/Disney_dollar). Disneyland prints Disney dollars and you can use them as if they were real money within Disneyland. Now why would people want to spend Disney dollars instead of real US dollars - because they are souvenirs, possibly will have some antique value and because they are fun. And you can trade Disney Dollars back to US dollars.
Similarly with Bitcoins, they are minted/mined by people using their computers. People - like you - are using their computers to create new Bitcoins and to authenticate Bitcoin transactions. And then people buy these bitcoins to use to spend on things. As to why people want to buy bitcoins - well, there are some who think that they are a hedge against inflation (ie. the Fed is printing too many dollars but there are only a finite amount of bitcoins), but the majority are buying to use to buy things anonymously. The main thing that makes bitcoins vaguely interersting is that they allow you to pay for things over the internet with a relatively high level of anonymity. So what things can you buy? Mostly shady things - no offense to anyone, I believe it's the truth. You can use them to buy illegal things on Silk Road (google it), you can use them in online gambling, and you can use them to pay for things that you don't want traced back to you - like usenet accounts, and VPN connections (used to bittorrent things... mostly pirating movies and music). And then there are a variety of legal non-shady things.. but I bet that most of the use is for semi-shady things.
So that's it. The one thing that everyone gets stuck on is why they are worth ~$100 each. The reason for this is much more philosophical and gets into why things have value. They are worth $115 today because people buying them are willing to pay $115 for them and the ones selling them are happy that they are selling them for $115. As to whether this is too much or too little, it's debatable... Back it 2008 the price of rice quadrupled in a matter of a month or two because of really flimsy reasons - lots of people wanted it, not many were selling it. (
http://en.wikipedia.org/wiki/2008_global_rice_crisis ) Things have value because people assign them a value and, as a group, we agree to this value. They have value because people give them value. Right now, if you wanted to buy something on the internet and not have the payment tracked back to you, your easiest solution is to buy some bitcoins to use for this.
So that's where the money comes from.
I will say that I'd be surprised if you could make much money with a 7770 at 200MH/s. Have you done the calculations? Unless your electricity is super cheap or your computer is super efficient, you are probably losing money over time due to the price of electricity.