As far as the value goes, find a commercial storage facility nearby and base your charges on their rates (but probably a bit less).
The easiest way to discover how and why the committee member is given this privilege is to ask the secretary or strata manager or secretary and if they don’t answer, put a motion on the agenda of then next EC meeting.
But be warned, storage is at a premium in apartment blocks which means as soon as you establish that it should be paid for, there will be a bun fight to see who is at the front of the queue to do so.
Turn it into a bicycle park if need be.