At least that's what I've observed over the past few years. Twitch was bought $1B with a mere 50M users. But highly engaged users with successful monetization. So it fits my model. WhatsApp was bought for almost $20B under the assumption they could hit 1B users. A 2x premium for being so large.
So roughly $5 to $20 per user. That seems pretty reasonable.
I wonder if and when people get tired of that. I also wonder how user engagement/retention numbers evolve globally over time; if almost everyone treats their users like cattle, I won't be surprised if more and more people started treating on-line services as throwaway.
 - corollary: all the "we're trying to change the world", or "our passion is to help you do X" they say on their page is just lies.
 - that also applies to paid services; I've read that classic Maciej's post about paid vs. free services, but companies aren't stupid; if being paid signals trustworthiness and leads to more users, they'll make the service paid
 - https://blog.pinboard.in/2011/12/don_t_be_a_free_user/