When you buy ads, they go directly into the bid-ask feed auction, just like stock market trades, and respond to real-time price changes. The cost of an impression depends on how much you’re willing to bid above the initial offer, which is set by the publisher’s minimum ask price, available supply, and how much advertisers are demanding within the ad exchange.
TLDR: Set the CPM at the bid-feed level to decide how much you’re willing to pay for an impression served.