You're not going to believe this, but they have also fallen by 97% per MWh.
Unbelievably, it was also 97% by GWh!
Yes and no. On a spherical cow, units convert without cost. In the real world, someone ordering GW-hours of batteries is buying differently from someone buying W-hours. The prices of batteries at various scales have moved differently, with small batteries having become cheaper faster than big ones.
Even more: in the old days, any buyer slurping up production by the MWh would have noticeably tipped the balance of supply and demand.