The rapid ageing of populations around the world is creating complex challenges for national governments. The establishment of sustainable and equitable long-term care systems for old and dependent people is one of the main issues of social policy in developed countries. The aim of this work is to define a cost model for residential and day care centres for dependent persons in Cantabria (Spain). The cost model will make it possible to establish the theoretical cost of attending to the needs of the different types of dependent persons in the different types of care centres, and the methodology used could be extrapolated to other regions. The daily cost per user for elderly residential care is €53.72. The cost per user in elderly day centres (5 days) is 32.56 euros. In residential centres for people with disabilities, the values range between €47.41 and €75.25, depending on the category of the centre. In three categories of centres, the public price is not enough to cover the cost (physical disability, intellectual disability, mental illness—low care); therefore, the administration should reconsider their public prices for these kinds of centres if they want to really contribute to the sustainability of residential care centres. This research will have important implications for policy makers in the context of the fulfillment of the SDGs and where better support for old and disabled people and their carers, as well as fair and efficient financing of social care services, are essential to address the current and future challenges of dependency.