How much should I be paying for language and Art tuition?

[fuckbucket]

My ddtwins, Year 8, are going to be getting extra help in MFL and Art from a qualified teacher (not teaching at their own school), which I am swapping for walking her dog. What's the going rate per hour? We are outside the M25.
TIA.

We live in the north west, and the going rate here is around £25 per hour.

Thanks. Getting the sums right for it being twins sharing a tutor is what makes it complicated.