Market forces determine the value of commodities. If something is rare and good it will attract a high price, but if goods are offered at a very low price, they are assumed to be valueless. Corinth was a trading city and market mentality was embedded into the culture. If a visiting lecturer charged a high price to teach new students, they presumed it reflected the high value of important knowledge. The problem was that Paul did not ask for money (Acts 20:34). So the Apostle had to defend the gospel, which is of eternal value, against critics of his 'free message of free salvation'.
 
But first Paul agreed the principle of workers being paid. It was the normal way to exchange labour for money that could buy food, clothing and shelter. God's care for the hard working oxen demonstrated that He agreed that people should be paid for their work (Deuteronomy 25:4). That same principle applies to spiritual ministry and gospel work: Paul said that such workers are right to expect support from other believers. But he had voluntarily relinquished that right; not because the gospel had no value, but because taking money for sharing what God has freely given could be misinterpreted as greed.
 
Jesus had every right to the glory of sinless heaven but chose to live in a sinful world and be crucified by sinners, so that repentant sinners might be brought to glory (Philippians 2:7). That is grace. Paul knew how easily money could undermine integrity and hinder the gospel, so he was determined to earn his own money to support himself and those he called to help him (Acts 8:18-23). It was important that the gospel of grace should be free (Matthew 10:8), because Christ had already paid the price for it with His blood (Ephesians 2:13).
 
It is right to support people who work in God's Kingdom so that they have enough to meet their needs. But pastors and missionaries who are greedy for money bring Christ's gospel into disrepute. Alas, some believe that they must be seen to be wealthy, to prove that their teaching is true. Others are simply grasping and still others are caught up in an addiction to money. In communities, which are so money-conscious (they always want more or never have enough), it is often better to forgo the right to be paid and give yourself freely to the work of ministry, trusting the Lord to provide and being willing to earn money or use personal resources.