Background: From a health service perspective, informal care is often viewed as a potentially cost-effective way of transferring costs out of the formal healthcare sector. However, informal care is not a free resource. Objective: Our objective was to assess the impact of alternative valuation methods and key assumptions on the cost of informal care. Methods: Informal carers who assisted in the care of a head and neck cancer survivor for at least 1 year were sent a postal questionnaire during January–June 2014 requesting information on time spent on caring tasks in the month prior to the survey. Time was costed using the opportunity cost approach (OCA; base-case) and the generalist (GRCA) and specialist (SRCA) replacement cost approaches. The impact on results of how household work and informal carers not in paid employment are treated were investigated. Results: We estimated a cost of €20,613 annually in the base case (OCA – mean wage) for informal care. The GRCA and SRCA equivalent costs were 36% (€13,196) and 31% (€14,196) lower, respectively. In the extreme scenario of applying a ‘zero’ opportunity cost to carers not in paid employment, costs fell by 67% below the base case. Conclusion: While the choice of costing method is important for monetary valuation, the sociodemographic and economic characteristics of the underlying population can be equally so. This is especially important given the heterogeneous treatment of older carers, female carers and carers not in paid employment in the OCA. To limit this, we would suggest using the SRCA to value informal care across heterogeneous carer populations.