One day the two brothers decide to sell all their sheeps to make money. Each sheep costs as many dollars as the original number of sheeps. Then they share the money in ten dollar steps: first, the elder brother gets ten dollars, then the younger brother gets ten dollars, then it's the elder brother's turn again, and so on, by turns, until it's less than ten dollars that remains for the younger brother. Then, in order to make it a fair share, the elder brother gives his penknife to the younger brother.
What does the penknife cost?