The majority of exorcists are part-timers, and spend the rest of their work hours doing the typical priestly duties of performing funerals, administering last rites, hearing confessions, etc. The men of the cloth who do decide to undertake this extra-credit sort of mini-career can expect a certain amount of bonus pay, although that differs greatly from state-to-state, country-to-country and even parish-to-parish. It also depends on the financial position of the one possessed and how badly they need their freedom.
That being said, consider this: A certain Reverend Bob Larson, who says he’s performed more than 15,000 exorcisms (for an undisclosed price), charges $10 to take a questionnaire on his web site to see if you might be possessed. On his site you can buy any of his many books, purchase his two DVDs (as a boxed set), set up personal time with Reverend Bob (he charges airfare for cross-country exorcisms but also uses Skype) and purchase a beautiful replica of Reverend Larson’s Cross of Deliverance (a $100 “gift,” shipping/handling not included). The good Reverend also holds more than 60 seminars each year (though he doesn’t divulge what it costs to attend).
So yes: In theory you can get rich being an exorcist, though you may want to skip that whole “poverty” part of the priestly oath.
Another thing to consider is the tax incentive: Any sort of treatment that is required for on-the-job injuries (acidic vomit, spike impalement, blindness, loss of faith) might be—in-part or completely—tax deductible under the current tax code. Check with your tax adviser.