Hi,
As for I am aware, there is no pricing difference between mutable and immutable. All considered as VM and you pay for the runtime you leverage the resources.
For SAP:
Example - form the azure calculator, you can select Linux operating system flavors with SAP components. Like RHEL, SUSE. - This will give you only SAP instance cost but there are other components like storage and networking and etc, those sets need to be considered based on your SAP design.
I am not a SAP expert, just giving my opening based on my experience from previous design works..