I came here because it’s not fully clear in all the documentation that 1 subscription = 1 Twitter user. Worth updating the Premium API documentation to make that unequivocally obvious.
Secondly, looking at the pricing, it’s seems a bit confusing how this could ever be affordable for 99.9% of Twitter apps.
The pricing for user subscriptions seems to be a non-starter:
$339/month for 25 users = $13.56/month per user
$2899/month for 250 users = $11.60/month per user
Take for example Twitter management app Hootsuite: they charge $45/month, and you can connect 10 social profiles.
If Hootsuite wanted to use webhooks for real-time DM/reply notifications, and a customer had 10 Twitter accounts connected, it would cost Hootsuite over $100/month in costs to Twitter, while only earning $45/month.
Even if Enterprise pricing for tens of thousands of user subscriptions brings the price per user down, the initial price/user at 25-250 levels ($11-$13) makes it seem unlikely that price/user would drop to affordable levels (e.g. $1-2/user/month).
The same comments were made when this was announced:
At this level, it looks like only a handful of enterprise-priced (i.e. $500/month+) Twitter products would be able to afford to use this API.