TL;DR: If you’re not prepared to invest $300–$400 (in cash or time) to make your first $1,000 in royalties, your odds of success are the same as hitting a casino jackpot. ----- After four years in publishing, the most common question I saw is, "I published my book, when will the dollars start piling in my account?" Authors see a few books hit the stratosphere and assume it's just luck. A magic trick. It's not. The authors who consistently outperform everyone else aren't wizards. They're business owners. They understand that the "KDP" in Amazon KDP doesn't just stand for "Kindle Direct Publishing" – it stands for "Knowledge", "Dedication", and "Planning." The good news? Their success is just math. And you can do math. The 30-40% Rule: The Math They Don't Talk About You're browsing Amazon and see a book in your niche that's consistently ranked. You use a royalty calculator and your jaw drops. That book is pulling in $10,000 a month in royalties. Here is the secret you're not seeing: That author is very likely spending $3,000 to $4,000 a month to get those royalties. That's the rule of thumb the pros know: A successful, scaled book often reinvests 30-40% of its royalties directly back into marketing. This isn't profit they're throwing away. It's the cost of doing business. It's the fuel. That money goes directly into Amazon Ads, strategic Facebook ads, TikTok content creation, Instagram community management, and newsletter placements. They aren't making $10,000. They are spending $4,000 to acquire $10,000. Applying the Math to Your Goal Let's set a more common goal. You want to generate a consistent $1,000 in monthly royalties. Congratulations, you now have your marketing budget. Using the same 30-40% rule, you must be prepared to invest $300-$400 per month to make $1,000 a reality. This is the number that separates hobbyists from professionals. The hobbyist sees that $400 as a "loss." The professional sees it as the investment required to generate a $600 profit and build a sustainable author platform.