I'd be careful about generalizing this article to business Ph.D's.
A graduate assistant at Yale might earn $20,000 a year for nine months of teaching.
The norm in business schools is about $30K. If you teach, you are paid extra per semester.
The average pay of full professors in America was $109,000 in 2009—higher than the average for judges and magistrates.
Not in business school. A full professor easily makes over $150K for a 9 month appointment, not to mention additional monies from consulting and teaching in executive education. For example, professors can make $50K for a week of teaching exec. education and charge $5K a day for consulting with industry.