Everything has a cost and the laborer is worthy of his hire!
Whoever produced the material needs some sort of means in which they may meet their financial obligations otherwise they will need to do something else instead of writing or creating content.
There is also cost involved for publishing, printing, marketing, warehousing, and distribution. Even if you received the materials for free, there was someone who shouldered the cost.
Should you do so for monetary gain or in order to become rich? It should not be your primary motive but one should not feel guilty if God chooses to bless your efforts and reward you monetarily.