The material given on MathCity.org is free and publish to help the students to learn Mathematics. It will take a lot of time to mange this website and prepare new material to publish on this website. The MathCity.org generate its revenue from Ads and Donations. The big problem is not to generate revenue but to get new and updated material to publish on MathCity.org.
From May 2008, MathCity.org has adopted a new policy in which MathCity.org will pay money to a person who will participate to update/run MathCity.org to encourage the peoples to be a part of MathCity.org.
MathCity.org will pay the amount according to the material and demand of the material on the website. You can ask to Administrator of MathCity.org for the worth of your material. Remember that MathCity.org is a non-profit organization and it will never sell your material, it will just publish to help others.