It’s probable that Ken and Roberta realized from close to the outset that they could make a living from this. Steven Levy’s book Hackers contains a lively account of the early days of Sierra. He writes (about Mystery House):
Ken and Roberta, in a fit of optimism, had bought a box of one hundred blank disks at the nearby Rainbow Computing store, and once the fliers were sent to computer stores and the ad placed for a reluctantly paid two-hundred-dollar fee in the May 1980 issue of a small magazine called MICRO, they waited. The phone rang, on that first day in May, and then there was a break and then it rang again. And from then on, it would be a long time before Ken and Roberta could count on their phone not ringing.
Ken and Roberta made eleven thousand dollars that May. In June, they made twenty thousand dollars…