You’re absolutely right that this will be very difficult to balance. It doesn’t need to be perfect and probably can’t ever be absolutely perfect. There are however 2 major variables missing from this calculation that will make it way easier to balance it. Favour costs on both sides of the equation. You can directly manipulate the relative power of each faction by adjusting the rate at which you can interact with them. If MG is a clear front runner then we can slightly reduce the cost of prophecies and slightly increase the cost of interacting with the Bazaar. These also get broken down further because we can adjust these more granularly. Each Prophecy has its own cost, we can adjust different item types in the Bazaar and we can adjust buying and selling independently.
Together, these form a pretty complex system. Part of why we have this more complex system is so that we have these dials to turn to be able to fine tune the relative power between the two.