The Cost % (shown in both the Organic Positions & Advertising Positions reports) is based on the overall Traffic Cost shown in the top left of your report. You will notice a Costs % for each keyword the queried domain ranks for. This represents how much of the overall cost being displayed is coming from a given keyword.
For example, if I search capitalone.com and want to understand how much the keyword “best credit cards” costs this domain to rank in AdWords, I can look under the Costs % column. As you can see in the screenshot, “best credit cards” has a Cost % of 1.49%. This means that an estimated 1.49% of the overall $3 million traffic cost is coming specifically from the keyword “best credit cards”.
Please keep in mind that these numbers are estimations and are not actual. The Traffic Cost & Cost % metrics are estimations based on the advertising behaviors analyzed by SEMrush.