The cost of repairing a transmission fluid leak depends on the severity of the leak and how you choose to repair it. For small leaks, adding brake fluid to your transmission fluid should stop the leak, and you can buy brake fluid for around $5. Alternatively, you can use a stop leak product for small leaks, and this is available for $6. This product is specifically designed to stop transmission fluid from leaking. If the leak is larger and you need to take it to the garage, then the price would vary from garage to garage and where the root of the leak is located. Most garages will give you a free estimate.