The paper analyses the economic effects of specific co-existence measures (like e.g. increasing isolation distances between GM and non-GM plants, changing flowering time between varieties, non-GM buffer zones) in maize crop and seed production in selected regions in France. The results are based on simulations of agronomic measures and corresponding cost calculations for specific measures and landscape simulations. In maize grain production levels of adventitious presence below 0.9 % can be achieved without any co-operation between farmers in neighbouring clusters. In case fields are located in the same cluster, additional measures have to be taken with very differing cost effects depending on the specific characteristic of the measure. Additional measures should also be taken (either by the seed company or by the commercial GM grower) for ensuring co-existence between commercial GM fields and non-GM seed production clusters, while current practices would be sufficient for ensuring co-existence between GM and non-GM maize seed production plots for a threshold of 0.5 % adventitious presence.