I have doubts that the Walton Abduction case is accepted as fact by so many people as you suggest. There is good reason to have serious reservations about the veracity of the story, and when I've shared those reservations, more often than not, people tend to re-evaluate their own belief in the case. As for whether or not Walton has made money off his story, that alone isn't all that relevant, but when taken in the context of the other issues at the time, it doesn't look good, and even if the gross revenue over the years hasn't been substantial, that doesn't mean there hasn't been any effort to make it more substantial, or that it has been so meager as to have been irrelevant, or that the notoriety alone hasn't been a motivating factor.
Few people sell movie rights, write and promote books, make public speaking appearances, take part in TV documentaries, appear as a guest on radio shows, claim rewards from a national paper for their story, go on game shows where they're almost guaranteed to collect even more money basically just for showing up, even if they fail to demonstrate they are being truthful, and then finally create an annual conference to "commemorate" their event and charge money for attendance, if they weren't actively interested in promoting their story and reaping any associated rewards.
And I'd hardly call all this activity "low key", especially in light of the fact that all these efforts have made Walton an iconic figure in UFO abduction lore. I just don't understand the level of denial that seems apparent in the "true believers" of this case. There are simply too many reasons to have reservations about it, but hey, it seems that no matter what anyone says, the "true believers" are going to be that way regardless of what anybody says, and all one needs to attract the "true believers" is enough exposure. And Walton has had plenty.