Start your next discussion with 1 Corinthians 9:11: "If we have sown spiritual seed for you, is it a great thing that we reap a material harvest from you? If others share in this rightful claim on you, do not we still more?"
Paul says that even though he would have been justified in being paid for his ministry (v. 18), he chose to forego payment in order to eliminate a potential source of criticism from his detractors. He explains in verses 14 that "the Lord ordered that those who preach the gospel should live by the gospel."
Other verses to examine are 2 Thessalonians 2:6-10, Romans 15:26-27; and 2 Timothy 2:6.