There are no clear rules or procedures to determine travel expenses. Usually, guests and guides decide it as they want on chat.
However, in order to prevent trouble, guides and guests have to decide the cost as they agree with each other.
In preliminary consultation, you need to check all about expenses of a trip: what is included, what guests pay for in advance, what guides pay for and so on.
It is all up to you but it must be decided with no discrepancy with guides.
Huber recommends that guides make everything collected and paid in advance. In that case, guests only need to pay for unexpectable things such as lunch and souvenirs. "Cash-less, cash first" will lessen money-related troubles and make you feel more like you travel with friends.
If you do not understand about expenses, please feel free to ask your guides.