You pay each time a user clicks on an SEM result. You pay nothing when a user clicks on an SEO result. SEM results are paid placements, and your brand is charged each time a user clicks on the result. Therefore, you must have a budget for continually showing SEM ads and using this form of PPC lead generation. On the flip side, you are never charged when a user clicks on an organic search result.
It might be awkward, but please don't scroll past this. This Monday we humbly ask you to defend Wikipedia's independence. Our 2019 fundraiser will be over very soon. 98% of our readers don't give; they look the other way. If you are an exceptional reader who already donated, we sincerely thank you. If all our readers donated just $2.75 today, Wikipedia could keep thriving for years. Most people donate for a simple reason—because Wikipedia is useful. If Wikipedia gave you $2.75 worth of knowledge this year, take a minute to secure its future with a gift to the Wikimedia Endowment. Show the volunteers who bring you reliable, neutral information that their work matters. Thank you.