Answer:
They worked only for a share in the profits to be made by the owner of the lands of the sharecroppers. To be calculated, 'of course', after costs. The owner had no interest at all in showing a good net profit, since that would mean big pay-outs to the sharecroppers. And since he was keeping the books, it was fairly easy for him to calculate (or make reservations for) costs in such a way that the profit - and the pay-out - always was minimal.
Many owners kept their sharecroppers in a state of virtual slavery by putting them in debt to him: through bills run up in the store and saloon owned by the boss, by the rent he charged for their homes and by the money he asked for use of heavy equipment. In a 'bad' year, there simply would be no pay-out and income for the sharecroppers and the unpaid debts would pile up. And even if there finally was a good year, the owner could - and would - offset their pay-out against the money still owed to him.
Hopefully this helps out!