This is the problem. Income and cost-of-living is a spectrum and varies wildly from place to place. And everyone wants to put some kind of hard line on tax rates, etc. To me, this just underscores that income tax, as a concept, is completely fucked. You can offshore your earning and shelter them if you're savvy enough. If you earn your income from capital gains, you're paying lower rates regardless, from whatever percentage of those gains happen to fall in your particular jurisdiction.

A federal sales tax with exemptions for basic necessities (food, rent, school supplies, etc.) would make so much more sense, as already exists in 7 states at the state level.

In this way you don't have to worry about whether income was earned offshore or onshore, if you're buying stuff or services here or importing stuff, you get taxed on it.