Income and expenditure of applicant on visit visa

[Hello.pak]

Hello.
My parents and sister are going to apply for visit visa to UK. My father is the sole earner of the house so while filling application for my mother and sister what should he write in 'how much money you spend per month?'. This amount should include living costs, dependent costs, rent or mortgage and bills if any. My father bears all their expenses as well. So he should write nil for them and state the whole in his application?

Please help.