Budget a school or class field trip.
A field trip has fixed costs that do not care how many students come — coach hire, staff cover — and variable costs that scale per head. That structure means cost per student falls sharply as numbers rise, which is why a half-full coach is the main driver of an expensive trip. The supervision ratio is reported alongside because it is usually a policy constraint rather than a budgeting choice.
Cost structure
Total = coach hire + staff cost x staff required + entry fee x students
Per-student cost
Cost per student = total cost / students attending; supervision ratio = students / staff
Because transport is fixed. A 420-unit coach across 48 students is under 9 each, but across 20 students it is 21 — the single largest lever on trip affordability.
It depends on age, activity and jurisdiction, and typically tightens for younger children and higher-risk activities. Follow your institution's policy rather than a general figure.