I did my MBA essay on burnout, especially with managers. I did my research with a questionnaire and statistics in an hospital.
My conclusion was that when in a work place the expectations are most of the time higher than what people can give, you create a work environnement that in short term becomes more and more stressful. That is evident.
Unnecessary stress becomes rapidly unproductive. You pay for sick leave benefits for example. That is very costly.
What helps a worker to avoid burnout is social support from the boss, co-workers + autonomy in the decision making. In other words, if your employees feel in charge of their situation (empowerment) you will avoid many burnout case.
But nowadays, many high level managers do not simply care about if an employee break down or not (you just have to replace him or her). But if you do a tight cost inquiry, you will always find out that, in long term, it is much more costly to manage that way. Also hiring and training employees is the most expensive task in an organisation.