Deciding whether to stop accepting cash when fewer customers use it but some rely on it

Cash now accounts for a small fraction of our sales, but a few regular customers rely on it. Going card-only would simplify closing the register, reduce counting errors and make banking easier. On the other hand, I don't want to exclude people for a modest operational saving, and local cash-acceptance rules may apply. Has anyone calculated the actual labor and error costs before changing policy? I'd rather make a careful choice than follow a retail trend.