The notion of surplus value loses all meaning after more production factors are introduced. What most people don't get - Marx was adept of classical political economy fully in the tradition of Ricardo ans Smith, and they all recognized only manual labor as production factor. But 5 years after the first book of Marx all this become obsolete. Marshall & co. introduced modern concepts into the economics - additional production factors, marginal value theory and much more. Marxism died being 5 years old. All we had after that was a zombie held alive by a few power hungry morons.