Well you have a nice little problem, so with out any pictures I am asuming that it is a very labor intense job, time consuming and difficult because of the heights and obstacles you have to go thru to remove the metal that being said the only way to make this job a profitable one is to charge for your labor. And that of a helper this type of job requires to have at least two men on the job at a minimum for the simple reason of safety. We don't think about safety until something unforeseen happens that's why it's called a accident, any way judging from the property owners dicesession he is not in a hurry because he does not want to spend any money to do this demo. You don't want to start this job and realize that you are not making money and have a negative attuitide about this project, so the answer to how much to charge is what you have to decide, your labor plus the helpers labor plus unforeseen expenses
As far as the scrap money goes I would split it with him down the middle this would offset his costs. Good luck hope this helped you.