The cost of changing clutch plates in a Royal Enfield Thunderbird 350 can vary depending on various factors, including the location, the service provider, and the specific clutch parts required. Additionally, prices can change over time due to inflation and fluctuations in the cost of spare parts. To get an accurate estimate, it is best to contact authorized Royal Enfield service centers or local mechanics in your area. They can provide you with a specific quote based on the current prices of clutch plates and the labor charges for the replacement.