I have not embarked on this journey (in spite of a few nudges from people around me), but I have worked closely with some judges before, and it is not easy. They are constantly studying and sitting more exams at both national and ISU level. There is little money involved; they might get a token amount towards expenses and no more for most competitions. I think that if you judge a Grand Prix or above, the LOC pays expenses, but no more than that.
It takes many years of study. You need to trial judge at tests and competitions, and there are certain tests, I think. I am not sure how the process works exactly in the US. Of course you can say no to any competition, but the less available you are, the less likely it is that an LOC will ask you again.