There is no perfect metric to determine how much a contributor deserves, but there is another simple solution: let each member choose how much of the project's income they take! This is how Liberapay teams work, and how they used to work on Gratipay (they're working on bringing that feature back).

If you want to know more, the second part of https://medium.com/liberapay-blog/a-new-platform-to-fund-wor... is a short introduction to our teams system and contains a few additional links.