Per-user, per-device, per-site is the easy half
The recurring side of an IT budget should be boringly predictable. Multiply your per-device fee by your device count. Multiply your per-mailbox fee by your mailbox count. Add the per-site fee for each office. That number is your monthly base — it should change only when your headcount or footprint does.
Hardware refresh: the half that surprises everyone
Workstations should be replaced every 4–5 years. Servers every 5–7. Firewalls and switches every 5. If you're not replacing 20–25% of your workstation fleet each year, you're either getting lucky or you're going to get a big bill later. A good MSP gives you a rolling schedule and the cost in next year's plan.
Project work: the rest
Network refreshes, server migrations, M365 cutovers, compliance prep — these are line items, not surprises. Talk to your MSP every fall about what should be on next year's project list, and what each one costs. If it's on the calendar, it's not an emergency.
Three numbers to put in next year's budget
Recurring monthly × 12. Annual hardware refresh budget. Project allowance (start with 10–15% of recurring). Add those three together and you have the IT line for the year. Review quarterly. Adjust slowly.