Are the voice lessons coming from the choir teacher? When I was in hs you had to go to lessons, they were required, an hour a week. I took private voice lessons outside of school and they were $20/hr (she's now up to over $30/hr).
I went to school for vocal performance, took voice thorughout college as well as sang in select choirs, but ended up getting my degree in history instead of voice. Total I have about 2/3 of my music degree done...missing upper level theory classes and upper level electives. Because of this, I only charge $15/hr. That's really cheap. So you are getting a good deal. However, I'd want more information about the private teacher's qualifications, references, observe a lesson. There are a lot of teachers out there that are not good singers and don't have a good understanding of vocal techniques. My hs choir teacher was a prime example. Sweet lady, amazing pianist, but not at all a vocalist. She would be someone I wouldn't pay $5/hr to take a lesson from.
If it's someone good and your daughter wants to do it, the price is really good if it's an hr lesson. I would also check though and see what they plan on teaching. Are they working with her on solo pieces and her technique or will they just be working on the choral rep? If it's choral rep, waste of money. They should be working on her personal vocal technique. It should also not be a group lesson. It should be completely private, to focus on her.
It does stink that they require it but are charging for it. That seems a little fishy.