Less than you’d think.
The cost depends, more than anything, on the time it takes. There are no short cuts to a good read. As a rule, to write a non-fiction book of, say, 75,000 words is going to take me four months; more, if serious research is involved, considerably less, if it’s an article or a short punchy guide.
You’ll be commissioning a skilful published writer who has bills to pay like everyone else, so a book will cost several thousand a month, payable in stages. Professional results can’t be achieved in a matter of weeks, and if the cost is going to worry you, the project won’t work. You’ll be stressed and the trust between us will melt away.
Otherwise, contact me.