The gender pay gap and the gender work gap
There are lies, damned lies, and statistics.

Here are three incontrovertible statistical facts about employment in the U.S. today:

1. Women earn about 82c for every dollar men earn.
https://en.wikipedia.org/wiki/Gender_pay_gap_in_the_United_States

2. Women work gainfully about 6 hours 50 minutes for every 8 hours men work gainfully; equivalently, women work 52 minutes for every hour men work, or a 34-hour week for every 40-hour week men work.
http://www.statcrunch.com/5.0/viewreport.php?reportid=7996

3. For every extra hour worked per week, salary rises by about 2% (as of 2000). Five extra hours worked per week is equivalent to 10% higher salary.
http://www.nber.org/digest/jul06/w11895.html

(See update: This is salary, not wages per hour.)

Conclusion: U.S. women today earn, on the average, about 8% higher salary per hour than their work is statistically worth, as compared to U.S. men.

Update:
I was not precise about the influence of longer work hours on salary (I confused per-hour salary vs. total salary). Apparently this relationship has been changing in the last 30 years, and now 50 hours of work per week pay about 5% more per hour than a standard 40-hour week.
https://hbr.org/2013/11/defend-your-research-working-long-hours-used-to-hurt-your-wages-now-it-helps-them/

So, item 3 is not relevant, but instead this should be used:
4. For every extra hour worked per week, wages per hour rises about 0.5% (as of 2010).

Conclusion: U.S. women today earn, on the average, about the same wages per hour that their work is statistically worth, as compared to U.S. men.