I'm a young parent, so my personal experience might be relevant because it's not too dated. In my final year of high school, I carried 6 IB credits, played on 3 varsity teams, was first chair violin in the senior orchestra, head of the senior choir, and did recreational yoga and downhill skiing teams. It can be done, but it gets busy. I remember Fridays being long--6:00-8:00am at the pool for practice, orchestra rehearsal at lunch, cross country training from 3:30-4:30pm, and fencing afterwards to 6:30pm. I'd get home, eat dinner, then sleep fall asleep! I wouldn't have done anything differently; the extra-curriculars were the exciting part of high school. I usually had a spare period every day, which was when I finished my work for the day, or (if I'd finished my work in class) I'd just goof off in the senior lounge with friends, often both!
I attended a private high school, so all practices were on site except for swimming, and transportation to and from competitions was on the school's bus. The pool we used was en route to my mum's office, so she headed into the office early. My mum was (and still is) incredibly supportive!
The cost was mostly subsumed by tuition, and the fencing gear was provided by the school. All I had to provide was a bathing suit, swim cap, goggles, running shoes, skis (which I already owned), and a violin (which had been my father's childhood instrument, so $0). I already had to have track clothes for phys ed, and I didn't need any special clothes for competitions because we wore uniforms.
