I constantly see customers with so many managed disks which are unattached and orphaned. Recommendation here would be to delete these if you know you can. Else (from a VM within Azure in the same region where the disks are (to save on egress costs)) use Azure Storage Explorer, download the managed disks as VHD disks, then copy to an Azure Storage account and mark the storage account as Archive (tape storage backend).
Archive storage is estimated less than 10% the cost of managed disk storage. Note, VHDs can be brought back and imported again as managed disks at any time if they are needed.
Pricing can be confirmed by using the Azure Pricing Calculator
There’s a lot of solid guidance in here.