A high idling issue in a BMW 525 can be caused by several factors. Here are a few possible reasons and potential costs associated with fixing them:

1. Idle Control Valve: The idle control valve regulates airflow when the engine is at idle. If it becomes dirty or faulty, it can cause a high idling issue. The cost of replacing an idle control valve can vary depending on the model year and location but could range from $150 to $400.

2. Vacuum Leak: A vacuum leak occurs when there is an unintended air intake into the engine, leading to a high idling problem. Fixing a vacuum leak might involve locating and replacing the broken vacuum line or repairing a gasket. The cost could range from $100 to $300, depending on the source of the leak.

3. Throttle Body: The throttle body controls the amount of air that enters the engine. If it gets dirty or experiences electronic issues, it may result in high idling. Cleaning the throttle body could cost around $100, while replacing it can range from $200 to $500.

4. Mass Air Flow Sensor (MAF): The MAF measures the amount of air entering the engine to determine the correct fuel ratio. A faulty MAF sensor can cause a high idling problem. Cleaning the MAF sensor can cost around $50, but replacing it might range from $100 to $400.

These are common reasons for high idling, but there could be other potential causes depending on the specific symptoms and vehicle condition. It's important to have a professional diagnose the issue to ensure an accurate repair and cost estimate.