It's not that simple. Many offshored jobs are "blended" with jobs onshore. For instance, with call routing through IVRs, the next available agent is given the call, whether here or in India.
The cost of a project may involve programming offshore with project management onshore.
Your tariff scheme would be an absolute nightmare, both for the companies trying to track the source of their services, and the auditors who were trying to enforce compliance.
First off, it is NOT a tariff scheme! Its a commodity tax that EVERYONE pays equally.
As far as tracking and auditing, well how do those companies get paid today? Billable hours, additional programming or consulting, whatever they invoice for services here will get a portion deducted for government overhead. Maybe we can call it an invoice tax? I'll have to think about that a little more. Either way there will be a lot of accountants who used to track income looking for work also.