Not necessarily (though sometimes).
Mostly it's a combination on our group trips of:
Owner-pilot looking for some to help fly their aircraft (and pay their share of costs)
We spend time with them explaining the function of the crossing
We get to see who is good/effective/enthusiastic/capable/safe
We can add them to our database
So, yes, they pay for the experience but if they're good it's a win-win.
In any and all events, it's an incredible trip - regardless of whether they do more later or not.