I think it's hard to generalize. The $650/month is for private music lessons for my two girls (on top of the lessons they get in school), tae kwon do lessons for one and dance for the other. In addition, there are extra fees associated with those activities. Dance recitals run us about $350 for tickets, costumes and fees. My older daughter is in a regional youth orchestra which has a variety of concert fees. There are belt testing fees and tournament fees for tae kwon do. My oldest will be going for her black belt soon, and the testing fee for that is $500, on top of the $110 monthly fee. There are always additional expenses that crop up.
We send our kids to some summer activites as well. We don't have to pay for childcare in the summer, but our kids go to camp for a couple of weeks (about $200/kid/week), take swimming lessons and other things like that. If you need camp all summer long, that's very expensive.
Activities, are of course, optional. Many people will spend more and many will spend less. For us, it is important to nurture our children's outside interests, but other people don't or can't do that, or choose not to.
I don't know that girls are more or less expensive than boys. I'll bet it costs more for clothing, makeup and accessories on average for girls. On the other hand, auto insurance will be more for teen boys, and they'll eat more. I've got nephews who play very expensive travel sports, but so do some girls.
We have excellent medical and dental insurance, and our kids have good teeth and have been healthy. If your kids need braces, that can run $5-6K/kid. We are lucky in that respect, but my dd has a friend with a serious disability that has cost her mom a great deal of money.
You'd probably get a more accurate answer by looking for articles on the topic, where people have done research, rather than just looking at anecdotes. One number that I've heard thrown around for teens is about $10,000/year, which I think is pretty reasonable, but again, you can do it for more or less, depending on your kid and your circumstances.