Be aware that the coin locker charge will probably be for 3 days, since they usually charge per calendar day.
If your onward flight from Sapporo is domestic, you might consider forwarding your luggage to your destination in Japan.
If your onward flight from Sapporo is international - or connecting to an international flight - you may find it cost effective and convenient, to arrange a van-style taxi for your party, and store the luggage at your hotel.