Chiefly the time of the process owner and the manager who can describe how work escalates, plus retrieval of change histories that may sit in older document systems. If the procedures are poorly versioned, expect additional effort to reconstruct who decided what and when.
Fix the record before paying to explain it
Where change histories are thin, some of the spending goes into building an account of the past rather than presenting one. That is worth knowing before agreeing a scope, because it is avoidable in future cases if the organization starts recording revisions properly now. Ask the adviser to identify which gaps genuinely matter to the claim rather than reconstructing everything the department has produced.
Add one item that belongs ahead of everything else in the budget: a short assessment of the qualifying history before any reconstruction work begins. A career in an operational specialism frequently contains a contracting period, a secondment to a customer, a transfer between group entities, or earlier time in the United States, and that arithmetic can change what is possible. Buy that answer as its own small engagement, before anyone spends weeks rebuilding change histories.
Hypothetical example: a concrete mix design specialist's employer commissions three weeks of document reconstruction and only afterwards assembles the employment timeline, discovering an eighteen-month period invoiced through a subsidiary that raises a question the reconstruction cannot answer and could not have been expected to.