Productivity
A famous report put together by Gallup, titled "State of the American Workplace" revealed that 70% of employees are disengaged at work. This costs companies between $450 and $550 billion dollars every year. That's 'billion' with a 'B'!
Looking at data from 1.4 million employees surveyed by Gallup, they discovered that the top 25% most engaged teams had 21% higher productivity than the rest.
Turnover
There is a direct correlation between the level of engagement and staff turnover. When the first goes down, the second goes up. If engagement goes up, turnover goes down. In other words, you get to keep your best and happiest employees.
In the same study, Gallup discovered that the same highly engaged companies saw 25% less turnover (in high-turnover industries) and 65% lower turnover (in low-turnover industries).
If you think about this for a moment it simply makes sense: High-performing companies hire employees who are a good fit, thus decreasing turnover rates.
Absenteeism
Engaged employees actually love going to work every day. Even if they feel a little under the weather, they will try to power through and make into the office. On the other hand, disengaged employees will look for any excuse to "call in sick". Actually, businesses with highly engaged team members saw a drop in absenteeism of around 41%, according to the Workforce Institute. Another study by DDI World showed that "...in a Fortune 100 manufacturing company, turnover in low-engagement teams averaged 14.5 percent, and absenteeism hovered around 8 percent. For highly engaged teams, absenteeism was only 4.8 percent, and turnover came down to 4.1 percent."
Onboarding and Training
Finally, training a new hire and getting them up to speed is extremely costly. Depending on the complexity of the job, a new hire will only be a fraction as productive as the person they replaced. In fact, a good rule of thumb says that full productivity is only achieved after 90 days. Add to this lack of productivity the actual cost to train the new employee, the loss of productivity from whoever is training the new hire and all the costs associated with actually hiring the person in the first place and you can see how retaining an employee is far more cost effective than getting new blood into the company.