base-2 or base-10 system when reporting semantic model sizes on Power BI Service?

Well, I don't wanto to argue anymore but here it's not just the case of storage pricing. Of course, they look at it as a priority and they still have enough space.

However, they need to show the KPI to workspace owners so that they realize how much space is occupied by UNUSED semantic models. 

There is no point in occupying the space, however cheap, if the datasets are unused and they clutter the environment unnecessarily. It's easier to declutter on a regular basis and teach the owners to discontinue unused content. Besides, they believe not only in cost effectiveness but also in environmental issues and it's their mission to avoid accumulating unnecessary data using up IT resources.