At Green Park we have a hand picked pool of senior interim executives who we know personally - people who have demonstrated their worth, time and again, in successful client assignments. These executives are confident and competent enough to link their cost to a series of key performance indicators.
Please explain the emboldened parts in a simple way.
(Not a teacher)
This is a somewhat odd phrase (I'm not sure what "cost" means here) and I wonder if the meaning might be that the executives justified their high salaries (their cost to the company) by means of a series of key performance indicators.