In most cases, royalties are the most lucrative aspect of idea selling and can earn inventors a substantial amount of money with a licensing agreement. The amount of royalties paid will depend on a few things: the company that buys your idea, how many units you sell, and the percentage you receive as part of your contract. For example, if you have 2% royalties on your invention and the product costs $20, you will earn $0.40 for every unit sold. Many inventors choose to sell their ideas to companies that pay royalties rather than a one-off payment, especially if the concept is especially promising, as royalties can often end up allowing them to earn much more over a longer period (passive income). As a result, many companies offer this as an option to inventors.