This 'break the price down' nonsense gets right on my t1ts. Fair enough if she wants a price per room, but beyond that no chance. You can't get the price of a new sofa, tin of beans or whatever broken down into , materials, transport costs, shop labour charges, stocking, wastage, profit etc. You'd also get some funny looks if you asked her to break down where/how she got the money to pay for the work. The job costs £xxx to do as a whole house, it costs £xxx for this room and £xxx for that room etc.
The other point about breaking down your costs is who is going to have access to that information? There's nothing to prevent the 'customer' passing it onto a relative or the next plasterer in to quote.
So for me the answer is always "This is the price for me to do the work as described.".