SIL (hairdresser) has very kindly offered to do my colour for me at home. I've kind of outgrown my usual salon.
She is insisting on only being paid for the cost of the colour - she suggested doing it at home rather than the salon she has a chair at as it would cost upwards of £80 (what I would usually pay)
Obviously I can't just pay her for the colour, she's doing me a huge favour!
Question is: what proportion of that £80 would she make on average so I can pay her appropriately?
She's refusing to tell me! 
Thank you in advance.