Big A,
An awful lot has to do with what local labor bill-out rates are. In my area it's $90 to $120 per hour.
Your customer, in my opinion, is getting a very fair deal.
My bread and butter is work in existing dwellings. 15 hours feels reasonable. From your description, I'll bet there was one overloaded circuit covering the second floor and now there are three distributed circuits, and grounding on top of that. Your work will have modernized, significantly, the second floor.
Regardless of what you get paid, the customer gets a permanent increase in the value of their property, and an electrical system that is more useful.
