I think one starting point to discuss the sizing calculations for ReFS could come exactly from the blockstat tool that Timo wrote, and the value it shows. The main split I see is between blocks that are stored only once, so they actually belong to the file, and anything that belongs to at least two references, which means we are saving space. So an information about the file could be about these two parameters, exclusive (consumed) space + shares (consumed) space. Probably the sum of all the exclusive space of a chain is the value you want to bill a customer for, plus the entire value of a full backup file which is the starting point and probably has most of the shared space.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1