The CPI is usually extracted out from CPM, and you are actually charged cpm(theres a couple that charge cpi/cpc). So since a free app has a much higher install rate from the view, it lowers the cpi.
The idea is for 100k they'll put youre add in front of 800k users, and 80k will install if its free. but only 1k will install if it costs money. numbers totally made up.
The ones that do charge cpi, if your conversion rate isn't high enough that theyre profiting[since they're paying per impression or could show an ad more likely to convert], theyll just kill your app and not advertise it.
The idea is for 100k they'll put youre add in front of 800k users, and 80k will install if its free. but only 1k will install if it costs money. numbers totally made up.
The ones that do charge cpi, if your conversion rate isn't high enough that theyre profiting[since they're paying per impression or could show an ad more likely to convert], theyll just kill your app and not advertise it.