I asked grok and here's what he said :

>Flat Fee
Set a fixed price based on project tiers (e.g., basic edit vs. full integration).
Predictable for client
$100–250 for basic adaptation; $300+ if it includes print-ready files.

>Value-Based
Price based on the project's impact (e.g., 5–10% of expected book revenue).
Maximizes earnings for high-value work.
If the book could earn $5K, charge $250–500.