What the numbers mean
A $75,000 salary in Seattle gives you the equivalent of $46,296 in purchasing power after accounting for state income taxes (0.0%) and the local cost of living. The same salary in Austin gives you $61,983 — a difference of +34%.
Median rent in Austin runs about $1,700/month — roughly $400/month less than Seattle's $2,100. That's $4,800/year back in your pocket before you factor in anything else.
These numbers use a cost-of-living index where 100 = national average. Seattle's index of 162 means it costs 162% of the national average to maintain the same lifestyle. Austin's is 121.