We've been implicitly having a discussion over email about when and when it does not makes sense to offer an honorarium to someone who comes to Michigan to give some sort of seminar or perhaps more than a seminar. I had never really thought about this systematically before so the discussion has been interesting as well as perhaps useful - though as we have come to no clear agreement in the case at hand perhaps it has lacked in practical usefulness.
Here are some thoughts based on my own experience:
1. Regular seminars at peer institutions at institutions in North America never include an honorarium but do include travel, lodging and meals. Usually the dinner is a nice one (where sometimes "nice" means the best local BBQ joint if it is a place in the south and I suggest that as an alternative to the local upscale continental cuisine place that duplicates what I can easily get in Ann Arbor.
2. Big picture / policy talks at universities and institutes do usually include some sort of honorarium, in addition to travel, lodging and meals. I think the idea here is to partly compensate for the labor of having to write a new talk, in contrast to a regular seminar, where I am also receiving, ideally, valuable comments on an existing paper.
3. If you come for visit that is longer than one day and do something more than just give a regular seminar, this typically includes an honorarium (unless the point of the visit is actually to hang out with your co-author friend in the department). UWO has a program for "short term visitors" that was exactly like this where the visitor was expected to be in residence for two or three days, to give one regular seminar and one subject area overview seminar aimed primarily at graduate students, to be available to meet with faculty and graduate students and to participate in meals. Here I think the honorarium is intended to compensate in part for the time and trouble of preparing the "subject area overview" talk, which one would not normally have sitting around ready to go, and in part for having more meetings with graduate students than in a typical one day seminar visit. Feedback on graduate student meetings with outside visitors can be really useful for the faculty for many reasons.
This all seems pretty reasonable. How should honoraria be thought of in terms of compensation? That may depend on the person receiving the honorarium. For junior people (or for theorists not working on marketable topics like auctions) the honorarium may actually come close to the relevant consulting rate in some cases. In others, it will be quite far from it. Yet honoraria typically vary by task and not by person, probably to avoid various moral hazard issues and also a lot of internal debate about whether or not some potential visitor is sort of famous or really famous or whatever. Of course, if everyone understands how the system works, no one gets offended.
At the margin, an honorarium may help to attract big names with busy schedules as well. Michigan has an endowed lecture series called the Woytinsky lecture. Jeremy Bulow of Stanford was just here last week and gave a very fine lecture on bidding strategies in spectrum auctions. He received the a nice honorarium (which came nowhere close, I am sure, to what he is paid for a day's work consulting on spectrum auctions) - I think the same honorarium that Woytinsky speakers always receive (which has, of course, been declining in real value over the years). This all seems very reasonable too.
Now, what do you offer if you invite someone to come for two days and give two regular seminars in different venues with differnet audiences? Is the rule the same as for one regular seminar, as the speaker is not doing any additional preparation and is receiving useful comments on two papers rather than one, or is it more like a short term visit? I lean towards the latter but it seems that people of goodwill may differ on this.