Spanish authorities have proposed providing external resources to substitute for non-renewable groundwater withdrawals in the south-eastern Murcia Plateau, which would require building new water infrastructure. This article assesses the supply costs and the economic impact of two alternatives, one based on supplying desalinated seawater and the other based on supplying water from the Tagus–Segura Transfer, with an innovative scheme to exchange it for desalinated seawater. It is found that farmers cannot afford the cost of either alternative. Authorities face the dilemma of either strongly subsidizing the external water supply, beyond what seems reasonable, or facing a disproportionate impact on the local economy.