As little as possible beyond the assessment itself. Records retrieval and legal review of the chronology are the sensible first expenditures, because a shortfall found here saves the entire cost of building a knowledge case and a corporate exhibit set that would never be used.
Ask for the chronology review as a separate step
Counsel can often price an initial history review independently of full preparation. That structure lets the business make a small decision before a large one. Ask what the review covers, what it would need from human resources, and what happens if the answer is unfavourable.
Retrieval costs from archived or third-party payroll systems are worth checking before assuming the records are free to obtain. Ask what the review would cost if the records turn out to be complicated, not only if they are straightforward, since the second scenario is the one that arises when leave and frequent travel are already known to be in the picture. A fixed fee for the review with a stated scope, plus an hourly rate for retrieval work beyond it, gives the business a predictable first decision.
Where the answer is unfavourable, the money spent has bought the avoidance of a much larger expenditure, and framing it that way at the outset makes the decision easier to approve internally.