CWI's probably average $50-$60K, but some with contract consulting work might clear $100K.
I think you can take the test using D1.1, ASME, or API codes. API is supposed to be the easiest. Some people say the ones they have worked with in the past are easiest though.
When you get your cert, it is good for any code. So, you would be qualified to work anywhere. Some people might ask you what you tested on, but it might not matter to them. Work experience would be the overriding factor.