This paper develops a stylized model of cross-border balancing. We distinguish three degrees of cooperation: autarky, reserves exchange and reserves sharing. The model shows that TSO cooperation reduces costs. The gains of cooperation increase with cost asymmetry and decrease with correlation of real-time imbalances. Based on actual market data of reserves procurement of positive and negative automatic frequency restoration reserves in Belgium, France, Germany, the Netherlands, Portugal and Spain, we estimate the procurement cost decrease of exchange to be (sic)165 million per year without transmission constraints and (sic)135 million per year with transmission constraints. The cost decrease of sharing is estimated to be (sic)500 million per year. The model also shows that voluntary cross-border cooperation could be hard to achieve, as TSOs do not necessarily have correct incentives.