Has she got some funding/scholarship from them or are their fees rather modest compared to other US institutions?
Most of the top US schools would seem to have higher fees than the 9k per year charged by the UK universities but also seem to have plenty of funds and will seemingly subsidise people for a variety of reasons from academic achievement to race and parent's income. I remember reading about a girl in the UK, straight A* student, rejected from Oxford but on the bright side given a free ride by Harvard with a fully funded place.
People tend to pick up on the headline figures and think that applies to every university and every student.
In-state tuition at state schools is usually cheaper than England, and that includes many top university like UC Berkley etc. As you point out many of the private school have relatively high list price, but the average student will pay no where near this as the actual costs will depend on their family income. If your family can afford to pay $50K pa tuition then you may well end up doing that at some select private schools but that isn't universal.
Then there is the fact that the headline figure typically includes full accommodation, food, transport, utilities etc.
Overall it is very easy to get better and cheaper university education in the US compared to England, especially if you move states to somewhere with excellent state schools.
Even at full cost private school, e.g. where my wife teaches it is around $55k pa, most of the students upon graduation will walk in to a job paying $100K