When a professor is invited to give a talk somewhere (especially out of town), he's often given an honorarium, in addition to having his expenses paid. Naturally, the practice as well as the amount varies, likely from field to field and institution to institution. Nonetheless, it's a common enough practice.
One doesn't need to be a great economist to recognize that this increases recipients' willingness to give talks, especially when it involves a good deal of travel. Yet when I first went into teaching, and for several years afterwards, I thought that this effect wouldn't be that great. We're in this field to spread our ideas, after all, and giving talks is a good way of doing that; what's more, many people, myself included, find it to be a good deal of fun (and, sometimes, something of an honor). It's also a good way to get to know faculty members at other schools, to get good ideas from questioners and commentators (espeically when the talk is at a faculty workshop), and in many instances to visit friends in town, though of course that depends on the town. What's more, while I'm not wild about spending time in crowded airports and airplanes, I find I can get a good amount of work done when traveling.
Having small kids, though, exposed for me an interesting economic twist on the matter: The pleasure of spreading ideas, and the honor of being a featured speaker at a high-profile event, are valuable, but they're valuable to the invitee. The costs of the trip, though, are often imposed on the invitee's spouse, who has to mind the children alone.
Naturally, we can assume that the spouses love one another, and derive at least some joy from knowing that the other is doing something fun, and some unhappiness from knowing that the other is being stuck with the kids. Still, "Honey, I'd like to go out of town for a few days; can you mind the kids? It'll be a great way for me to spread my ideas, I'll enjoy giving the talk, and the invitation is something of an honor" is a harder thing to say than "Honey, I'd like to go out of town for a few days to give a talk, and they'll be paying me $X,000." So even if an honorarium's marginal value to the speaker is not that significant, the honorarium's marginal value to the speaker's spouse may be the deciding factor.