Most WADS have some sort of bidding or auction system in place. It helps determine who gets their ad shown or served first. Sometimes it's strictly bid price. Others have quality metrics involved.
Most CPM WADS that are used for retargeting, show exactly what your ad spend will be. You decide what your bid will be, but if someone has their bid set higher, their ad will show instead of yours. Then it's a simple matter of monitoring clicks and adjusting bids accordingly.
Search engines like Google and Bing will show a suggested bid price, but don't start your bid at that level. What I suggest with search engines is to bid slightly higher, say 10% more than the top bidder.
So how do you know the real bid price? At the time of this writing, it's a secret that few people know about.
While doing keyword research, you might have noticed a little link to download a CSV file of the keyword phrases. Download that CSV file to your computer, because it contains info not displayed anywhere else.
Note the bid price. This is the exact top bid price someone is actually paying. All you need to do is bid slightly higher. Then with all things being equal, like quality metrics and ad to lander relevance, you'll snag top spot.
The way I see it, if you want the rainbow, you gotta put up with the rain.