I would agree that DDS/DMD education is expensive. However, I would tend to believe that an MD education is most likely more expensive not only in monetary terms but also for a variety of tangential factors.
I took an econometrics class before, and one thing I learned is that rate of return for a general dentist is greater than that for a primary care practitioner (specialists earn tons more, don't count them). Even lawyers and businessman would earn more than MDs in the long run if we were the consider the amount of money they earn minus the total amount of schooling plus the investment of that money. Remember, you should not only count the amount of money charged for schooling but also discount for the amount of money you could have earned if you did not attend school (So if you lost $60K in expenses from one year of med school but could have earned 50k, that's already $110k lost, without even considering the interest).
As for the other "tangential" factors: the stress of school (medical is definitely far more stressful than dental school--being extensively exposed to the cadaver, having to treat very ill patients, the excessive amount of memorization and length of education including residency), loss of youth (especially at the prime of life, the 20s), etc.