How are extraordinary expenses usually divided?

Under Ontario law, extraordinary expenses are usually split in proportion to the incomes of each parent. In your case, you indicate that the mom makes less than 1/10th of what the father makes. Under the normal rules the mother would only be obliged to pay extraordinary expenses in that same proportion, i.e. less than 1/10th of the total expenses. However, the parents of a child are entitled to agree to share extraordinary expenses in a different proportion. However, the father’s proposal for a 30/70 split seems to be unfair and significantly out of balance, in light of the described income levels.