O king, as soon as Duryodhana was born I told thee,—thou shouldst abandon this one son, for by abandoning him thou wouldst secure the prosperity of thy hundred sons,—and by keeping him, destruction would overtake thy hundred sons, that gain should never be regarded highly which leadeth to loss. On the other hand, that loss even should be regarded highly which would bring on gain. That is no loss, O king, which bringeth on gain.

In plain words: Vidura reminds Dhritarashtra that when his son Duryodhana was born, he advised giving up this one son to save the other hundred, since keeping him would destroy them all. A gain that leads to loss is not worth much. A loss that leads to gain is not really a loss.