For regular activities, like backpacking trip, river rafting etc. our adult leaders pay for their share of the trip however sometimes we are reimbursed for gas.
For summer camps since this is a week long activity, the troop normally shoulders the cost. When we prepare the budget for the summer camp, the cost of the additional adult going to camp is figured into the budget. The budget includes the cost of the camp program, patches and other incidentals and allocation for additional leaders to attend the summer camp. The logic behind this cost allocation - the leader takes a vacation to help run the summer camp. It is just fair for the leader to participate and not pay for the program, after all we are stuck with the scout for a period of 7 days. I have been to 4 summer camps and the only time I had to dole out money from my pocket was when we did not have enough scouts going and the summer camp was out of state.
For ordinary trips, our leaders are willing to pay the cost of going on an outing whether it is a backpacking trip or a camping one.