With the XYZ (Apple, Microsoft) stores these days I wonder if a $39-99 desktop software can truly have success on these compare the the average < $9 app.
High-end B2B software can certainly be profitable, I would guess the $500-2500 software coming with support, integration, training etc.
If you have decent app that solves problems well and you have enough energy left to do some marketing then probably you can make a decent living. Or you have a lot of luck, then no problems at all. But if you hope that customers find you by themselfes, maybe you shouldn't bet on the business.
EDIT:
>naysayers or cheapskates
Didn't mean to imply that your wife was one. The thing is, most non-software people don't know enough about all the work [1] that goes into a custom software app, let alone into a shrink-wrapped product (which is much more, if done well). (Using the old term shrink-wrapped, though a lot of software is delivered over the web these days. Same difference :)
[1] Figuring out requirements / features to include, design, development, testing, testing, testing, ... , support, ... , marketing, ..., maintenance, ...