The General Social Survey (sociology's premier social survey - and of course, unlike Cookie, it's nationally representative) asks how many people have EVER cheated on their spouse. The average over the last decade is about 13%. If we divide that up by gender, it's 17% of men and 11% of women.

Now, eyeballing the data by income (the website won't let me collapse income categories), I see maybe a slight increase of infidelity with income - but nothing suggesting that 30% of rich women could be having an affair. Remember, the GSS question is about ever having been unfaithful. So I wouldn't give Cookie much thought.

Check it out for yourself here:
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By the way, one other point: Americans are becoming less prude in terms of homosexuality, premarital sex, and even teenage sex. The only trend towards prudery is for extramarital sex: 82% say it's always wrong. That's about a 7% increase from the mid-80s. Then again, the GSS began in 1972 (when only 70% said it was always wrong), so maybe we're not becoming more prude, but returning to normalcy...