Is it reasonable for a guest lecturer/class speaker to ask for a stipend? 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?
I would look at this from the perspective of the professor inviting you. Most of the people a university invites to speak are professors (or postdocs/ phd students) from other universities. For these, giving the talk is beneficial career wise so there is no additional speakers fee. But they have to travel to the university, very often from out of town. This means there is no easily available budget for paying you to speak but there usually is a budget for your travel expense. Usually this also covers inviting you for a meal. 

So I would recommend asking for travel expenses. They should definitely pay for your parking ticket and if you come by car there is probably some fuel money or something like that as well. If you are interested the professor will probably also offer to take you out for dinner afterwards. This should be easy from their perspective. Asking to be paid for the talk as such could be a lot more complicated. If you want that to make it worth your while be prepared that the answer might be no.