RepairPal Certified shops can diagnose and repair your 2012 Audi A6 Quattro. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
No formula for this -- you can't really know what the future holds. All you can prepare for is factory required maintenance, and wear item replacement -- which is always due to your specific driving habits (meaning tire wear, brake wear, etc).