I think that's wrong, and just a way for the company to save as much money as possible. A fair salary should be a % of the $ value you create minus local taxes and expenses needed for your position(equipment, software, training...).
Otherwise this isn't fair, this is just the bare minimum to retain employees in my opinion.
For example personally I don't plan in staying long in my current city/country. But if I accepted local salaries I would never be able to save enough to move comfortably.
Whereas someone with a salary calculated using your formula, but in a more expensive city, could move wherever he want easily, his savings would take him much further.
So I only accept to work remotely for companies in richer locations who are able to pay more.
Also local purchasing power usually doesn't apply to objects(electronics, cars...)