From two reputable exchanges (platforms) at every CPM price point, only 60 cents of every dollar goes towards working digital media. The rest can be considered "mark up" or margin taken by the middlemen.

1. July 2017 / Page 0marketing.scienceconsulting group, inc.
linkedin.com/in/augustinefou
60¢ per $1 left for “working digital media”
“Supporting details confirm that for every $1 the advertiser
spends, only 57 – 63 cents goes towards digital media.”
“mark up”
“working media”
“working media”
“mark up”