Auvik only bills you for core billable devices (Firewalls, Routers, Switches, and Cisco Wireless Cloud Controllers).The problem is, you can also unmanage a device (if it's not a important device). This metric will account for any core device then look and see if that device is managed or unmanaged and give you the correct COUNT of billable devices.

map_by_key(Devices[],DeviceDetails[],'id')[? attributes.manageStatus == && (contains(['switch','firewall','router','l3Switch'],attributes.deviceType) || [?attributes.vendorName == && attributes.deviceType ==])] | length(@)