Though $20 in 1913 was worth almost $500 today[1], the nominal gdp per capita[2] in 1913 was about $400, while in 2013 it was over $50000. So: $20 / $400 gdp per capita in 1913 = 0.05 or 5% of yearly income. $400 / $50000 = 0.008, or 0.8% of yearly income. Thus, purchasing power has increased since 1913. The equivalent of $20 today will buy you much more than you could get in 1913. (That includes electronics that didn't exist in 1913: radios, wind-up LED lights, cell phones, etc.)

[1] http://www.bls.gov/data/inflation_calculator.htm
[2] http://www.measuringworth.com/datasets/usgdp/result.php