The real coat of an AuPair is about 24k/yr.
For your required 25hrs a week, you are looking at a breakdown of $18.50/hr. At that price, I'd hire a nanny. AuPairs are an enormous pain in the ass to host. Lots of drama no matter how well you choose. Also don't underestimate the amount of food a 20yr old eats. My groceries doubled.