As the name implies, RTB operates on an auction model. You set the max bid (CPM) that you will pay for a placement and win impressions at $0.01 above the next highest bidder.
Here are the core reasons for what makes BigDNet great:
If “Advertiser A” bids $2.00 CPM and “Advertiser B” bids $1.50 CPM, “Advertiser A” will win the impression at $1.51 CPM (Or to put it more accurately, $0.00101 for that specific impression).
Although you bid at a CPM rate, you are actually bidding on each individual impression (i.e. $1.00 CPM = $0.001 per impression).
For a more comprehensive explanation of the mechanics behind the bidding process, read our blog post: Real-Time Bidding Essentials: How Bid Price and Frequency Cap Affect Session Depth and Impression Volume.