I'm not in academia currently, but a former professor from grad school has asked me a few times if I would give a talk to his classes based on my professional experience.
I have said yes the past two times because I like this professor and was honored that he would ask me; I also enjoy teaching, and felt that speaking to his class would be an interesting opportunity. But, I also looked upon it as a favor because as a non-academic, these things aren't going on my CV.
He's now asked me a third time, and I am wondering at what point it might be appropriate to ask for a small stipend. The presentations last about 30-45 minutes with questions, and I do put a considerable amount of effort into preparing for them, plus the time spent travelling to campus. I'm not sure if I would describe myself as more of a visiting speaker or a guest lecturer in this situation because the talks are less about me and more about the field I work in. The classes are at well-known law schools; by contrast, I have received stipends in the past for speaking at events at much smaller colleges.
Is it customary to receive a stipend in this situation, or should I consider this more of a favor to a colleague?