CPC/CPA pricing for /units-for-slot

[Ivshti]

Problem

It's currently possible to set CLICK pricing for a campaign with zero IMPRESSION pricing, essentially implementing a CPC campaign.

However, the price returned from /units-for-slot (see AIP31) is respected by the AdView as the final price, and it's used to sort the available bids.

Research

https://support.google.com/google-ads/thread/1452036?hl=en - using historic CTR
https://blog.rontar.com/behind-the-scenes-how-advertising-auctions-and-cost-per-click-work - same

Solution

Use the slot average CTR to calculate a per-impression price to shim the price value.

For example, if price.CLICK is 100 and the CTR is 0.01, the returned price will be 1

It's important to use a default value in case the slot average is not available, or there's not enough data to gather it (e.g. less than 2000 impressions).

Same can be applied for custom aquisition events in the future, except it will require the rate between that event and impressions.

*The challenge is that the Market doesn't know the CTR. We can store an expectedCtr in the targetingRules by using { set: ['expectedCtr', 0.002] } and update it via a script on the validator