I was with you up until the very end. Pay N times more for tuition, expect to earn N times more _on_your_degree_investment_ in the long run.
That doesn't mean your entire income will be N times higher.
For example if you spent $100k on your tuition in year 0, and the rate of return from that degree is 10% per year, and assuming you spend the returns rather than reinvest them, then your degree is adding $10k/year to your income.
But you're not living on $10k/year, you're earning $80k/year, $10k of which is attributable to your degree.
So to complete the exericse, if you had spent 2x on your education, you would be making $90k/year. Not $160k.