Use talcum powder to refill. Much cheaper. However my experience is that you have to replace the unit frequently as the wheel gets stuck.
Far better value is "Jinbutsu Chalk" It's shaped like a triangle. Box of 10 sells for $5 Canadian and I can do a number of kilts with one.
Also on the market is a mechanical chalk pencil. Works like a "Stadler Mars" drafting pencil. Initial cost is expensive but one can get refills at a reasonable cost.