On some organized tour operator's sites, I see tour prices listed with a +amount labelled as "kitty" or "trip kitty" like this one.
What is a "trip kitty"?
From the FAQ of that tour operator:
What trips may have a kitty and what is it?
On some of our Overland and Basix trips we operate a 'kitty' system for our travellers. The kitty is a cash contribution to a central fund and overseen by the travellers and the crew. It helps fund accommodation, camp meals or activities for the entire group. Contribution to the kitty is compulsory on some trips while voluntary on others - please refer to the specific Trip Notes for details. Kitty amounts published in the brochure may change. Current kitty amounts can be found on our website so please check for the up to date amount 48 hours prior to your trip commencement.
Sounds like a way to lie about true costs in the price lists and advertising.
GAP Adventures and Tucan Travel had a similar charge known as the "local payment," although both abolished it within the last few years.
Whether you call it a "kitty," "local payment," or some other term, the charge contributes to a petty cash fund for the staff, allowing them to make arrangements for tour expenses like accommodations or local transport. By paying this amount in cash directly to the local staff, instead of to the head office, the tour organization would save on costs such as wire transfer and currency exchange fees, and possibly ameliorate exchange rate volatility. By having a dollar or euro cache on hand (no pun intended), the local staff has greater flexibility.
As transactional costs decline, and as headquarters wants tighter control and reporting on how funds are spent, the necessity of such a fund seems to have declined. GAP and Tucan, two of Intrepid's biggest competitors, now add the local payment directly to the cost of the tour.
It sounds like an amount to cover fixed costs, and your individual contribution might vary, depending on how many people are part of the activity. For example, if a bus is needed, the cost is fixed and divided equally between all of the participants. If more people participate, the individual's cost should go down (until they have to hire a bigger bus). If a guide is planned, then the guide's fee can be paid from the kitty in the same way.