The Rebbe was asked about having boys and girls together if it is very expensive to have them separated. 

“The financial argument that it is more expensive to run separate classes for boys and girls, is no argument at all, as the matter vitally concerns the future of many children, and even if the future of a single child were involved, money could be no consideration, as our חכמים say, 'He who saves one life is deemed to have saved a whole world'.”

(From a letter of the Rebbe on the 4th Day of חנוכה 5715 )