@sim Maybe one of the Mastodon hosting entities charges per user. Other than that there's no direct per-user billing.
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Most instances have a flat rate hosting until resource use tops certain limits. Bandwidth, storage space, RAM & CPU usage are the usual measures. This makes cost-per-user follow a stair-step pattern.
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Adding a lot of users at once can bump up some of these measures, as can uploaded files and subscriptions to & from other instances.