> If you were living on $40k/yr in Kansas City, you could move to SF and suddenly be saving $80k/yr
No, because not only do we have a progressive federal tax system, but state/local taxes in San Francisco are higher than they are in Kansas City.
Just running the numbers for a simple case, say you are single and renting. You earn $80k in Kansas City. Based on http://dor.mo.gov/forms/MO-1040%20Fillable%20Calculating_201... it looks like $2100 is not taxed as a personal exemption and $5000 is not taxed if you pay at least that much in federal taxes (which we will; see below). $6300 is standard deduction. So deductions are $13,400, taxable income is $66,600. Per the table on page 25 of http://dor.mo.gov/forms/MO-1040%20Instructions_2015.pdf that gives us $3771 in MO tax. Note, not enough to make it worth itemizing federal taxes if you rent.
Federal taxes for 2015 would give you a $4000 personal exemption and $6300 standard deduction, for taxable income of $69,700. That's gives a federal tax of $13218.
Total taxes are $16,989, for after-tax income of $63,011.
OK, let's look at San Francisco. Salary is $150k. Looking at https://www.ftb.ca.gov/forms/2015_California_Tax_Rates_and_E... the standard deduction for state taxes is $4,044 (less than Missouri, which is ridiculous given cost of living). Personal exemption is $109 (really?). Taxable income is $145,847. State tax is $11040. That's enough to make it worthwhile to itemize federal deductions. I didn't bother to look into the AMT rules for CA; this is meant to be a simple estimate. By definition the AMT can only make things worse here.
Federal taxes again have a $4000 personal exemption, our $11040 itemized deduction, taxable income of $134,960. Federal tax is $30860.
Total taxes are $41900, for after-tax income of $108,100.
OK, so say you were living on $40k in expenses in Kansas city, making $80k pre-tax, saving $23k per year. You move to San Francisco, start making $150k pre-tax, expenses go to $75k (if you accept the 1.87x cost of living difference). You are now saving $33k per year. More, true, but nothing even close to $80k/year.
Note, by the way, that the cost of living difference is largely driven by housing costs in this case....
> Would you really rather have $600k in Kansas City than $1 million in SF?
It really depends on my lifestyle. Still for a single filer, the Federal marginal tax rate on that extra $400k is 39.6%. The CA marginal tax rate is 12.3%. Medicare marginal rate is 2.35%. So overall marginal tax rate is 54.25%. CA taxes on that first $600k would be about $60k while MO taxes would be about $35k. So the extra $400k is actually about $160k post-tax.
This is the part where lifestyle matters: housing in SF can eat up a large chunk of that if you let it... If you're single, SF is probably a win in the 150k vs 80k case. If you have a family, chances are it's not.