You get fixed $ 0.25 a sale (it goes up to $0.33, $0.36 and $0.38 once you reach $500, $3000, $10,000 in total sales).
The rest is the risk and calculation of the agency. Theoretically if someone uses to full quota and happens to download all from a top tier ($0.38 per download) contributor, the commission would actually be more than 100%.