The cost of an electric vehicle battery depends primarily on its’ capacity. Global battery ‘pack’ prices are approximately £90 per kWh in 2022. For a Tesla Model 3 Long Range with a battery pack of 75kWh the approximate battery cost would be £6,750 to manufacture. From most market anecdotal evidence the price to supply and fit a replacement battery would be approximately £10,000-£12,000.
Batteries are based on proven technology, which means that they are a very reliable component of an EV. Many batteries can last a long time too and, if there is anything wrong with the battery, the manufacturer warranty should cover any problems. Most EV's have a 7 or 8-year (100,000 miles) warranty on the batteries.
For a more in-depth look at battery prices see Bloomberg NEF’s latest research on battery pack production: https://www.energy-storage.news/bloombergnef-average-battery-pack-prices-to-drop-below-us100-kwh-by-2024-despite-near-term-spikes