“Researchers have estimated the cost of losing a seasoned professional as 75-150 percent of the person’s annual salary,” according to Boris Groysberg, Chasing Stars: The Myth of Talent and the Portability of Performance (Princeton Univ. 2010) at 239 (See my post of May 14, 2005: estimate of about $100,000 for inside lawyer; June 15, 2005: upward revision of that estimate; and July 27, 2008 #2: 100-150% of salary of high performer with unique skills.). Those estimates may hold for most in-house lawyers, but not for the top lawyer.
When a general counsel leaves, the loss far exceeds such monetary ones as recruitment, relocation, training, ramping up. The entire ethos of the department alters. Passed over candidates can fester or leave; everyone has to prove themselves all over; established routines change; political elbowing sharpens; and everyone holds their breath (See my post of March 8, 2009: attrition in law departments, with 16 references and one metapost.)
My impression is that most general counsel leave on schedule. They retire with their boots on. Unexpected resignations or terminations – unusual, but they happen – occasion the greatest loss and upheaval.