I don’t know what to pay!

You can pay whatever you want, but if you are totally stuck here are some helpful pointers:

The modal average (most common) payment is £40 per month, the mean (numerical) average payment is £31.16 per month, and the median average is £30 per month.

The average of the lowest half of payments is £25 per month (lower quartile) and the average of the highest half of payments is £37 per month (upper quartile).

If you want to use a weekly payment amount, you can use the fact that I teach a 39 week year (same length as schools).