The https://learn.microsoft.com/en-gb/azure/update-center/update-manager-faq has a couple of interesting tidbits:
How is Azure Update Manager price calculated for Arc-enabled machines?
...
Following are the cases when Arc-enabled servers wouldn't be charged by Azure Update Manager:
- As additional value added to the Arc ESUs, patch management using Azure Update Manager for machines enabled for extended support via Arc would be provided at no extra charge.
- Arc-enabled machines present in subscriptions enabled for Microsoft Defender for Servers Plan 2 would be provided at no additional charge. For all other Microsoft Defender for Cloud plans, Arc-enabled machines would be charged by Update Manager.
Also PratheepSinnathurai, regarding 'When will the costs start?', the FAQ has:
I have been using Automation Update Management for free on Arc machines, would I have to pay to use Azure Update Manager on those machines?
We'll provide Azure Update Manager for free for one year (starting from when Azure Update Manager goes GA) to all subscriptions that were using Automation Update Management on Arc-enabled machines for free. Post this period, machines are charged.