Is it reasonable for a guest lecturer/class speaker to ask for a stipend?
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.