The “eCPM” used for admob mediation are just dummy numbers - they are essentially used to specify a hierarchy of which ad network to try first.
So setting eCPMs:
ad network 1 - $1 eCPM
ad network 2 - $0.5 eCPM
Will behave EXACTLY the same as:
ad network 1 - $3 eCPM
ad network 2 - $1 eCPM
This applies to Admob and the other ad networks you add to admob mediation.
It is called “eCPM” because presumably you would want to use the eCPMs you actually see for the different ad networks and would use those. However this is naive, since eCPMs vary greatly and can vary depending on how much traffic you give them, and where in your app the ad is shown etc. So PRACTICALLY you would try each one and get a sense of how each performs, and then decide how you want mediation to allocate traffic - and then will set the admob “eCPMs” to be arbitrarily something which will achieve what you want i.e. ad network 1 should get first chance to deliver ad - if that fails, then ad network 2 etc.
The only exception to this is the new “Admob eCPM floor beta” - which is another ad source (just like Admob banner ads or the other ad networks). With THIS SPECIFIC ad source, the “eCPM” you specify is ACTUALLY used to filter out the ads coming from this source to ONLY those which are EXPECTED to deliver eCPMs as you specified - NOTE this is not guaranteed but just based on what admob has seen for that particular ad’s performance. So in general if you set very high eCPM for “Admob eCPM floor beta” - for example $2.5 eCPM you will get higher eCPMs but the fill rate will drop to 10%-15% etc.
Admob House Ads are another source of ads - they do not cost you anything. You can specify a custom banner image, or construct one on the Admob webpage (using text and icons etc.). You can also I think point it to a URL. However there maybe a restriction on what type of linking you are allowed to do. Some have suggested on this forum that linking to other developer’s apps (using the Admob House Ads) is not allowed, and you can only link to your own app/url for app etc. Maybe someone can better clarify that situation …
When you decide to show Admob House Ads you will face the issue of “how do I ensure that Admob House Ads get shown” and not only the top eCPM ad source in your Admob mediation.
It turns out there is no way to specify a percentage as before - as all new accounts only have the eCPM option (some legacy admob Site/Apps may still allow percentage). But assuming you can only specify eCPM, there is a way to ENSURE that Admob House Ads get shown. And that is that you just specify Admob House Ads to have the second highest eCPM after the first one (for example Admob banner ads). Usually fill rate is not 100% but say 95% - this means 5% of the time your second choice will be referred to i.e. Admob House Ads if you have set that up that way.
In addition, I have seen that even the Admob House Ads do not have a 100% fill rate but more like 90% or something like that - which means you can setup AppBrain banner ads or something like that as the third highest eCPM in the Admob mediation.
This way you can get most traffic for Admob banner ads - then get a 10%-20% for your Admob House Ads and then 5%-10% for your AppBrain banner ads.
But obviously this only works because Admob is not delivering 100% fill rate for some things - so it just HAPPENS to work so you can effectively allocate a percentage to House Ads etc.
If you use Admob eCPM floor beta - you can tailor it slightly better - since setting a higher eCPM winds up reducing fill rate (practical observation). So by seeing how much traffic you are getting to your second highest eCPM (Admob House Ads for instance) - you can tweak the eCPM for the Admob eCPM floor beta to ensure you get (say) 40% traffic for your Admob House Ads.
But without a percentage allocation option, it is just this trial and error way of setting “eCPMs” until you get the right type of fraction-of-traffic going to your Admob banner ads, Admob House Ads and any other ad source like AppBrain banner ads etc.