With Android apps (and iPhone I presume), it seems that you pay once and automatically get any available updates. I'd guess that would work reasonably well for small utilities, so long as they continue to get new customers.
That hasn't always held true for some of the major iPhone apps, at least - when there's a major version release, they sometimes require a new purchase.
Also, iPhone and Android apps have the potential for ongoing in-app purchases by users. So you buy the GPS app once for a very low price (or free) but you have to keep buying the data updates if you're using it. Or new levels for Angry Birds.
Also, iPhone and Android apps have the potential for ongoing in-app purchases by users. So you buy the GPS app once for a very low price (or free) but you have to keep buying the data updates if you're using it. Or new levels for Angry Birds.