Identity is the component that is cheapest to adopt and most expensive to leave. The decision is usually made in an afternoon, early, by whoever is setting up the first environment — which is exactly the moment when the per-user option costs nothing, because there are no users.

00 / Two different shapes of cost

A managed identity provider bills per monthly active user. A self-hosted one bills per instance, per database, and per engineer. One of those lines slopes with your success. The other is nearly flat, and then steps.

For most B2B products the sloping line never gets steep: a few thousand seats is a rounding error next to the cost of running an auth server yourself, and the decision is not close. For a consumer product planned to be large, the lines cross. The only questions are where, and whether being wrong is survivable.

01 / Price it on the day, not from memory

Rates change, and so do the models. What is worth knowing in advance is the shape of the traps.…