[robgering]

This isn't exactly what you're asking, but consultancies that produce mobile apps on behalf of clients can see significant revenue. This is particularly true for apps that require user-to-user interaction or other functionality requiring a decent amount of backend code.

Unlike websites, which are largely commoditized by big players, custom apps still fetch large sums. The pros in this space charge project-based (or at least per diem) rates, so you'll need to be good at estimating your work.