Count the work, not only the users
Two firms with the same headcount can need very different support. Sites, servers, specialist applications and operating hours all change the workload.
Separate recurring work from change
The monthly fee should explain what is managed every day. Migrations, new offices and major upgrades are usually easier to control as separate projects.
A first audit improves the quote
Inventory and access checks reveal unsupported systems, missing backups and unclear supplier duties before they become surprise costs.