Laskern- in Israel (and also many schools in the US as well), BY girls wear a uniform to school (which needs to be purchased) but besides shabbos/yom tov, they still need clothing for vacation days (2 months in the summer) and many change when they come home from school as well so that they don’t dirty the uniform. So expenses for clothing for girls still adds up.
While a suit and hat or 2 for a bar mitzva age boy are expensive, a girl usually gets more shabbos clothing than the boy does, so it still probably comes out more expensive, especially as they get older.
It is an interesting question- I would say, for younger kids, boys are more expensive (think bris, more diapers if boys take longer to toilet-train than girls, then bar mitzva, and here in Israel, girls BY schools are with a few exceptions free, while boys’ chadarim/talmudei Torah charge tuitiion), but as they get older, girls get more expensive- clothing, seminary (way more expensive than boys’ yeshivos), and then of course the big one- wedding expenses and associated clothing and new household purchases, and often support/apartment for the married couple- although technically the OP asked only until marriage, since promises for support are usually given before the marriage, which are conditional on it, I am including it.