It's not a job you can price in a standard way. I was paid around $10,000 to ghost the auto-bio of a recording artist. Because people knew what she 'sounded' like I spent a lot of time interviewing her as well watching her Youtube clips.
My neighbour is interesting. He's a Tunisian Muslim married to a hot-lookin' blonde bombshell of a Lithuanian woman. They have no money but they're in love. I'm intrigued. I'd write their story for a percentage if they provided fuel by way of alcohol while I was working.
To write Trump's memoirs I'd require somewhere between $10 and $15 million.
The capitalist's answer is - as much as you can get. The memoirs of a non-celebrity are virtually worthless. Cousin Bob's uncle's story is unlikely to be mainstream published and will only sell 7 copies on Kindle.
It's a valid industry. The aged, those questioning their mortality want their existence, their achievements recorded for prosperity, l'est they depart from this Earth unnoticed, without leaving a mark. Memoirs are not normally subject to profit considerations - the subject is likely to die before any financial gain is realised. It's a personal negotiation: How much do you want to write this story vs how much does the subject want the story to be told.