The ECPM Proportional ranking model is now available. It’s an extension to the simple CPM model. In addition to using the burn rate for the campaign to bias the ranking, the ECPM Proportional model also includes the CPM price on the applicable contract.
Example, if Ad 1 has a CPM price of $5, and Ad 2 has $10, then Ad 2 will show approximately 2x more often vs. Ad 1 (assuming similar contract lengths and impression budgets).
You can read more about Ranking Models here