You want to claim that her expenses actually support your argument that she has income she is not reporting. That could be tough without an accountant and she could always lie about how she covers the expenses.
However, the bigger issue is the best interests of the child – money is only a small factor and the fact that she will lose money on the deal will not be very important to the court. Be prepared to show how you are raising your child. Plenty of people with little money do an excellent job of child rearing so I wouldn’t pin my hopes on just showing that she is in it for the cash.