My nephew is skilled (I think he is, he once had a business doing
landscaping) to build a 1.2m high by 7.5m long retaining wall. The wall
will probably be made of one two walls - a facing wall and a wall set
back, immediately adjacent to the soil. I'd say it needs to be about
30mm from front to back.

Lets say my nephew gets the materials at trade price, and I pay that
price for the materials. Of course, I'd want to pay my nephew a
reasonable and decent price for building the wall.

QUESTIONS: How much do you think I should pay my nephew per metre of
wall that represents a fair price, assuming he is not acting as a
business? What would be a fair price if my nephew were to say he would
do it acting as a business?

Thanks.