This question has been raised before, but here again the question is raised in the context of Catholic apologetics by Fr. Peter Carota, "Traditional Saints' Contempt for Fame and Money vs Catholic Apologetics Today" (TCP, September 18, 2013).
My parents were Protestant missionaries, and they were paid by their church's mission board. They also returned to the United States once every four years and went on "deputation," meaning they visited numerous supporting parishes, where they gave an account of their work and helped to raise money for the mission board. But they never took money from those they were involved in evangelizing.
Fr. Carota writes: "We traditional bloggers, YouTube-ers and speakers need to have great caution about becoming famous and earning money from our apostolate. Recently we have been hearing a good deal about the large salaries of Neo Cat. apologetic speakers. A little while ago I clicked on a headline of one of these speakers and was greeted with a plea for money. It went like this: “The summer times are slow and the money is not coming in as usual, so could you send $$$. It greatly saddened me, because I feel that this person does not need more money and it seemed as if he was using his apostolate to get money. I strongly believe that we should not be making money from converting people and saving souls." (emphasis his)
I feel a bit conflicted about this, because I remember as a former Protestant some good people who made their livelihood through their "ministries," just as there are some good Catholics also make their living through their apostolates. But I also remember some "free-lance" evangelical pastors of "independent" "churches," which they "planted" and "built" from the ground up, who made their money from preaching by which they drew in congregations. To people in their congregations, it probably seemed entirely normal, and when they gave of their "tithes and offerings" in the collection plate, they saw it as giving to the Lord's work. But I also see an obvious potential for conflicts of interest in such arrangements. The pastor can say he's accountable to God, but ... I've even seen such independent "churches" which send "elders" to visit the homes of newcomers with questionnaires about household income, who lay out a plan of regular "giving."
It may be one thing where an apostolate is producing books, CDs, DVDs, etc., for which there is a market willing to pay for these goods and services. On the other hand, as Peter, Paul, and Mary used to sing, "... if religion were a thing that money could buy, the rich would live and the poor would die." But of course, we all know that, don't we.
Food for thought.