That's a very reasonable price. He values his skills and time, maybe he wants to know whether you do. I ran my own business doing typesetting and other digital design work and was frequently asked by friends and acquaintances to do stuff I'd normally charge for. When I said it would take me x-hours and would cost £y on 'mates rates', they got very huffy and thought I was being unreasonable not to do it free. I was a single person, mortgage to pay and all the house upkeep, food, clothes, etc, finally I said no more – did they think Sainsbury's give me food for free – and stopped it.
Where else are you going to get the work done by a skilled craftsman for that price?
OTOH, I do agree with what others have said about the benefits he gets from your relationship. Typical man, really, he discounts the value of 'women's work' which saves him doing it, eating your food, doing his laundry, saving him some costs on energy, etc. I think you should point all that out to him and ask him if he'd reconsider charging you because you've already paid more than that. How he responds to you negotiating will tell you all you need to know about him, I think.
I wouldn't be prepared to tackle wallpapering anything bigger than a door panel, never mind a staircase wall, though, even if I weren't disabled, no way, paint only. Good luck with the negotiations. 