Cost is a red herring here - the real metric is opportunity cost. These are users who spend a lot of time on reddit and give up a ton of data, so they can almost certainly get a much better CPI from these users, and give them a ton of ads.
>>pclmul+(OP)
It is a gamble. I am a Reddit power user in part because of the superb unobtrusive experience of old.reddit and Apollo. If you shove it full of ads, it’s no longer the experience I got addicted to. At that point I’d rather leave.