One route to genetic adaptation in a novel environment is the evolution of ecological generalisation. Yet, identifying the cost that a generalist pays for the increased breadth of tolerance has proven elusive. We integrate phenotypic assays with functional genomics to understand how tolerance to a salinity gradient evolves, and we test the relationship between the fitness cost of this generalisation and the cost of transcription that arises from evolved differences in patterns of gene expression. Our results suggest that a salt-tolerant genotype of Daphnia is characterised by constitutively expressed genes, which does not incur a loss of fitness or a cost of transcription relative to a salt-intolerant genotype in low saline environments. We find that many genes whose expression pattern evolved in response to salinity are also involved in the response to predators, suggesting that the cost of generalisation may be due to trade-offs along other environmental axes.