Costs can be as much as 3x base salary for a mid-level manager. Formulas abound for calculating replacement costs. The Saratoga Institute offers a commonly used one. There are others that are less traditional. But any way you calculate it, turnover costs money.
Some turnover is probably unavoidable.
This is the kind of turnover caused by people getting married, moving to be nearer parents or children, employees leaving for medical reasons, etc. This is often called functional turnover and is the baseline below which a company rarely goes and can do little to change. Other turnover is generally considered to be preventable. Peter Hom and Rodger Griffeth, associate professors at Arizona State and Georgia State Universities respectively, have done extensive analysis of the literature and of past research into causes and cures of turnover. They have formulated an integrated model of factors that contribute to employee retention and turnover.