Here is the March Precept: Kind words do not cost much. Yet they accomplish much.

I think it means that it wouldn’t hurt to say something nice, like if you say something nice, it probably wouldn’t hurt or harm you. If you say something encouraging to someone it might brighten up their day or make them smile and that would make kind words effective; which means that the kind words did their “job” of making someone happy.

However, if you say something bad/mean , it would usually “cost” you something, like if you say something really mean to someone, they might get mad at you and maybe even do something bad to you which is the “price” for saying something bad.