When your at Cx City, take one of the buses from the road in front can't remember which number and take it to Tung Chung and grab the MTR from there into the city it is like a quarter of the price of taking the airport express and just as quick.
Guess it's more expensive to take the airport express cause people don't know any better, and it is better for those arriving at the terminal. But from CX city don't go to the airport and go in that way, go to Tung Chung and grab the MTR.
You can get anywhere from there, and try to have change, only a few of the automatic ticket machies take bills.
Hope that helps some