I disagree with this one.
The question is designed to legitimately get the poster, who is usually female, to recognise that childcare is a joint cost and as such, needs to be considered that way when it comes to deciding if it's worth working.
Absolutely the overall family income is relevant; but so many posters here say they can't afford to go back to work - they need instead to think of what they lose by not going back - independence, career progression, identity (for some).
It may make sense to stay at home, but it should not be predicated on the woman's income alone. It is also singularly unhelpful in terms of the dynamic created in the relationship - that the childcare cost is linked only with the mother working, not both parents.