One of the reasons Romney has 13% effective tax rate is that he gives a lot to charity - about 13% of his AGI on average, if you believe the auditors: http://content.usatoday.com/communities/onpolitics/post/2012...

So when using Romney as an example, it is good to know that if he did not give this much to charity, the rate would be somewhere around 27%, maybe more if charity deductions avoided triggering some higher rates, etc. Average effective income tax rate (don't confuse with marginal!) for millionaires is 23%, for "middle class" is 20% and below (the figures is for tax year 2010, other years I imagine they may differ).