The real cost is hours, multiplied
A two-hour session for forty people is eighty person-hours. That is the number to think with, and it is almost always larger than the catering invoice. It is also the number that should determine how much design effort the session deserves.
Eighty person-hours justifies a few hours of preparation. An activity chosen from a list in ten minutes and run without a debrief is not a cheap activity — it is an expensive one with the preparation omitted.
Three budget tiers, honestly
No budget. Most of the activities on this site are in this tier, and the constraint is not the limitation it appears to be. The things that most reliably change how a team works — structured peer consultation, expertise mapping, a pre-mortem, removing a pointless meeting — cost nothing but a room and a facilitator.
Low budget. Materials, food, a venue you do not already have. This tier buys atmosphere and the signal that somebody spent something, which is worth more than it sounds.
Moderate and up. External facilitation, travel, overnight stays, built activities with equipment. Worth it for genuinely large groups, for anything needing specialist safety knowledge, and when the facilitator needs to be someone with no stake in the team's politics — which is a real and under-rated reason to bring in an outsider.
Where external help is actually worth it
When the session is about a conflict the manager is part of. An internal facilitator who reports to one of the parties cannot run that room, however good they are.
When the activity has a safety dimension — anything on water, at height, with vehicles, with knives or with power tools. Pay for people who do this professionally and carry insurance.
When the group is large enough that one facilitator cannot hold it. Above about forty, you need a second pair of hands regardless of budget.
Not, generally, for a standard activity with a well-documented run sheet and a group under thirty. That is within reach of a prepared internal facilitator, and the preparation is a better investment than the fee.
Spending the attention budget
Teams have a limited tolerance for being gathered. Spend it on fewer, better sessions rather than a monthly obligation that becomes background noise.
A reasonable shape for most teams: one substantial thing a quarter, one short ritual weekly or fortnightly that costs ten minutes, and nothing else. The weekly ritual does most of the work; the quarterly event gets the attention.
If you have to cut something, cut the quarterly event and keep the ritual. That is the opposite of what usually happens and it is the right way round.