The axiom of balanced collective contributions is introduced as a collective variant of the axiom of balanced contributions proposed by Myerson. It requires the identical average impact of the withdrawal of any agent from a game on the remaining population. It turns out that the axiom of balanced collective contributions and the classical axiom of efficiency characterize the equal allocation of nonseparable costs, an allocation rule that is extensively used in cost allocation problems and in accounting. In particular, the equal allocation of nonseparable costs coincides with the nucleolus on the class of data games within the European REACH legislation. While our result does not hold on data games, extra axioms enable provision of comparable characterizations of the equal allocation of nonseparable costs and the Shapley value on this class of games.