Most parents (including on these forums) seem to act like it costs $0 for a high school or college student to live at home with parents, so they do not account for that when comparing costs between commute-to-local-college versus live-at-college, or whether sending a college student to live at college frees some money that would otherwise be spent on the student’s food, utilities, and commuting costs. Many parents may not have a good idea of what these costs are, because they are distributed across various household budget categories, rather than a category of “living expenses of student”. California public universities mostly seem to estimate “room and board” for students living with parents as around $5-7k, plus another $1-2k commuting costs, for what it is worth.
Because of this, from the student’s point of view, living at home and commuting to college may cost only tuition and books (because the parents are subsidizing food, utilities, and commuting by presenting it as $0 to the student), while the difference for living at the college is the full cost of room and board, rather than the difference between that and the true cost of food, utilities, and commuting costs at home.