Two dials, not one
Most CRM budgets assume a single variable: how many people sell. Platforms priced on contacts as well as seats have two dials, and the second one moves without you touching it. Every form fill, every imported list, every event registration adds records, and the bill follows. That is not a hidden fee. It is a pricing model, and it needs modelling rather than a single-month snapshot.
Why the entry number rarely survives year one
The starting quote is priced against today's database and today's feature list. Twelve months in, the database is larger and somebody has asked for a report or a custom object that lives one tier up. Both changes are normal success, and both are re-pricing events. Model them at the start.