ggirl, is that primary or secondary school?
At our primary school, woodwind and brass lessons are £40 a term, but that is for approx 20-25 mins a week, and often a shared lesson. It is also subsidised by the county music service, so not the true cost.
At secondary school level, or indeed any private teacher outside of school, I do not know of anyone charging less than £8 for 30 mins, and the norm is £10-12.