You may find it of some relevance that some church leaders can earn "free" travel for "leading" tours to the Holy Land or elsewhere by recruiting a sufficient number of paying co-travelers. If they can recruit paying participants beyond the minimum required to earn a free spot for themselves and a spouse, they can earn cash for the additional pax. Of course, the amount depends on the particulars of the trip (namely the marginal profit to the tour agency per additional pax), but a typical scenario might be one in which the tour price is $2700 per pax, and the pastor must recruit a group of 15-20 to earn free passage for two, and additional pax recruited might earn the pastor about $150-$175 each.

This suggests to me that under typical circumstances, the travel agency might consider your participation to be worth maybe 5% or so of the total revenue from the trip. If you have sufficient name recognition that they can put a significant premium on the price, then you might be able to get more than that.