My dh and I have struggled with this for a couple of years now. Is she missing out on other things she could be doing if she wasn't in the gym all the time?? What if we stop here and she really could have gone that far?? We finally decided that as long as she loves it, wants to go every day, and still has some time that she gets to just hang out with friends, we would stick it out with her. She gets up early to get her schoolwork done in time for gym. If she doesn't finish, she gets a shower and something to eat, and sits down to finish without me nagging at all. She has an incredible work ethic (I wish it would rub off on her brother!).
She is Level 10, training for Elite and is a Hopes gymnast. Her goal will be to make the US Junior National Team in the next year or two.
As far as cost, a lot. We have made many sacrifices to keep her in gym. Her siblings have their own interests, which we also support, but nothing comes close to the cost of gym. It cost around $10,000 last year, not including travel.