Less than you’d think.
The cost depends on the time it takes. 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. It always takes a bit of time and money to achieve professional results, and if this worries you, it won’t work. You’ll be stressed and the trust between us will melt away.
Otherwise, contact me.