Build it from the actual participant roster using current corporate per-employee charges, family charges and applicable ongoing obligations.
Show assumptions openly
The official site lists a US$2 million corporate gift per employee and separate processing fees; family participation adds its own terms. Add professional and other applicable expenses distinctly. Identify provisional participants and costs excluded from the company’s commitment so an authorised total is not mistaken for an all-inclusive guarantee.
Build the budget from the annotated roster rather than from the headline arrangement, and show the assumptions beside the figures. For each participant record the corporate per-employee charges as currently described, the processing fees, the family charges applicable to that person actual family, and any ongoing obligations attaching to the sponsor. Mark which lines are fixed by the programme as currently described and which are estimates.
Add professional fees for the individual advice each participant requires, since that is the item most often omitted from a group calculation and the one varying most between participants. Recheck the current terms before the budget is approved, because published terms may change and a figure carried from an earlier discussion may no longer hold. Present participants with unresolved issues as a separate contingency rather than inside a per-person average.