“But the prophet who presumes to speak a word in My name, which I have not commanded him to speak, or who speaks in the name of other gods, that prophet shall die.” (Deuteronomy 18:20)
Prophets should never require a monetary gift to prophesy, since the premise of genuine prophecy is that God has COMMANDED the prophet to speak. How then can we dare to refuse to set a condition on our obedience?
By the same token, when you encounter a prophet who doesn’t require a gift, that doesn’t mean you ought not to bless the man of God IF you CAN. Was the word right and true? Did it bless you, provide direction and insight? First be grateful to God for choosing to speak to you, and then be grateful for his servant who did not withhold the Lord’s counsel in an age when so many do. In such cases, is it good and right to be a blessing to the prophet.
Now, this doesn’t always mean money. You can be a blessing as an intercessor, by referring their ministry to others, etc.
Freely you have received, so freely give, according to your ability.