Dementia is a major economic challenge for societies worldwide. One approach of cost-of-illness studies focuses on the excess costs of people with dementia compared to costs in people without dementia. Yet, excess costs of dementia have rarely been examined. To contribute to the existing evidence on excess costs of dementia by adding information regarding the oldest-old. We used cross-sectional data from the AgeQualiDe study. This multicenter cohort study considers exclusively participants with a minimum age of 85 years. We included 856 primary care patients with (n = 154) and without (n = 702) dementia into the analysis. By using the reweighting method of entropy balancing, excess costs from the societal perspective were estimated as mean cost differences between both groups for each cost category of different healthcare sectors. In our primary analysis, total costs were 2.5 times higher for people with dementia (n = 154) compared with people without dementia (n = 702). People with dementia showed significant total excess costs of € 11,789 (95% CI € 8,343–15,234) compared with people without the disease in a 6-month period. Excess costs of informal care represented the highest cost category in both primary and sensitivity analyses. Our results emphasize the economic and practical value of informal carers in dementia care, as it is generally unpaid care provision saving expenditures for formal care. Health policy strategies are needed to foster community-based dementia care in order to relieve social security systems and to support informal carers of people with dementia.