I will tell you that this scam is very very common. More often then not it is not even a women on the other end of the computer. These scams often get worse, after you send the money they typically will claim they were held up by the government and need more money in order to come.
I know finding a relationship is not easy, but sadly to many people take advantage of good natured people looking for a good relationship. As for how can you know if it is for real.....the answer is you can not. Unless you know someone over there that can check on her and see if she is really who she says she is it is very difficult. Anytime someone wants you to pay for a trip to see you this is a red warning sign. She knew she did not have the money to come there......so why would she start a relationship with someone so far away?
Here is some interesting reading on the situation: