Je ne le ferai pas pour des prunes (fam.) = I shall not do it for nothing.

[Also: Je ne le ferai pas pour le roi de Prusse. This latter saying is said to have originated with Voltaire, who, after having been exceedingly intimate with Frederick the Great, King of Prussia, finally quarrelled with him. Both this King and his father, Frederick William I., were known to be exacting and miserly.]