https://repairpal.com/estimator
Check that link - enter your information and you can get an "average" cost for replacing a starter motor in your area on your BMW.
Naturally, have a proper diagnostic test performed to ensure you are not wasting your money.
RepairPal Certified shops can diagnose and repair your 2006 BMW 525i. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.