Speaking at last week's IT Directors' Forum, Susan Cuff, business development director for recruitment consultancy Best International Group, explained that the bulk of this cost is not related to recruitment fees.
It is comprised of factors such as loss of productivity and efficiency; management time; the learning curve of the replacement employee; and the impact of the departure, recruitment and induction of the replacement on your existing workforce.
Cuff stressed that if IT managers want to keep good staff they need to invest in various ways to ensure their IT departments are a good place to work.
They should take measures such as making sure they spend all their training budgets, encouraging coaching and mentoring, or providing flexibility.
Staff move primarily for career progression rather than for increased salary, she said.
"You can be outbid, but you can't be outbid on a great culture," she said. "People don't leave companies, they leave people."
Disturbingly for IT managers, she added that 28% of staff cite poor management as their reason for leaving.