Most Costly Method [C#]

[TheCPUWizard]

Consider the following:

public class SomeClass()
{
   protected virtual void SpecialWorkerMethod()
   {
       throw new Exception("Derived Class must Implement this Method");
   }
}

Can you see why the above just cause a client of mine SIGIFICANT money?

hint: The direct cause is directly related to C#, but the ROOT cause can happen in any language.