To show that their power came from outside of them, that it came from God, the apostles were given the ability to do what no mere man can do. They were exalted by Christ above all the power of the enemy.
The apostles were to do all these wonderful signs for the people freely, without cost. What was received was worth more than money could pay for. How much would they have given to be free from leprosy, to have their dead raise, and their beloved family members free from demon occupation? The gifts of God are beyond all price, even in the physical realm. But how could the apostles think of charging for the exercise of these gifts? They had received them freely from Christ: the gospel itself and the powers that they were exercising. How could that start acting as if these powers were their own and they were entitled to make a profit out of them? That would be a blasphemous abuse. But this is exactly what some do who make merchandise of the gospel and of their hearers (2 Peter 2:3). The Prosperity Gospel teachers add to this the additional deceit that in taking money from the people God will return a blessing the giver in kind, and by saying this they introduce a false covetous motive for what should be simple stewardship to the Lord.