SAM's default battery cost values were chosen to be roughly representative of battery costs for a project in the United States to help you get started using the model. You should review and change those costs for your own analysis.
Default battery costs are from the NREL Annual Technology Baseline.
For additional information about battery storage costs, see the following publications:
Feldman, D.; Ramasamy, V.; Fu, R.; Ramdas, A.; Desai, J.; Margolis, R. (2021). U.S. Solar Photovoltaic System and Energy Cost Benchmark: Q1 2020. NREL/PR-6A20-78882. (PDF 4.6 MB)