1) oScope — an oscilloscope in your pocket. https://itunes.apple.com/us/app/oscope/id344345859?mt=8
2) Octave — a real-time audio analyzer. https://itunes.apple.com/us/app/octave-an-rta-for-the-iphone...
3) Fourier — a spectrum analyzer. https://itunes.apple.com/us/app/fourier/id386084557?mt=8
I built all of these for fun in college, and I've occasionally updated them afterwards. The only thing I do now is answer a few emails a week.
I've since gone back to grad school, but the yearly income has not changed, and approaches my stipend (low 5-digit).
What's been really neat is how people have found unexpected ways to use the apps. Sound engineers for halls and communities use Octave to set up the sound for concerts. Teachers use oScope to help kids understand how sound is composed of moving pressure waves of air, and how pitch is the frequency of these waves. Also, oScope had a tiny cameo in the show Homeland, as a "fancy science-looking analyzer tool for spying on people" (uncredited, unfortunately).
Basically, it sucks to be stuck on the "passive" receiving end of a passive income project. Though actually, it's Apple's fault for not letting me trial software.
But I'm glad to hear these projects are working out and wish you continued luck!
As to whether you're entitled to support or feature requests, you can make an argument both ways. One could generate some word of mouth by following up on (simple) feature requests.
In one of the apps, Fourier, the reviewers say that the horizontal frequency scale is off by a factor of 2 (but the frequency display elsewhere is accurate). That's a small thing to fix that hasn't been fixed since Apr 2012. I think that at a $5 price point, my expectation is that the app is updated with at least bug fixes of that variety.
Second, I understand your frustration. I love building things, and sometimes I wish I could do that full time, but grad school is a 70 hr/wk commitment for me.
If the app is not useful for you without the missing feature, Apple does allow returns. I'm not quite sure how it works, but I see a few returns a year on my reports.
Also, I've open-sourced the hardest part of the app, the audio managing aspect, as Novocaine (GitHub.com/alexbw/novocaine), and along with the great package NVDSP, anybody could replicate the basic functionality of my apps with a few weeks of learning and effort. It'd be great for the audio app ecosystem, too!
I'm also interested in hiring a part-time developer to help flesh out the top-requested features, if anybody has ObjC coding experience. That'd make many more updates possible.
This thing also looks pretty cool: http://theamazingaudioengine.com/
$5 is worth 3 minutes of developer time at common hourly contracting prices. If a developer receives and reads an email from a customer, they will probably end up with no profit. If they respond to it, let alone are adding new custom features, they are certainly losing money compared to working for someone else.
It's nice to hear both sides of the story.
One thing that I've always wanted it to do is to be able to horizontally move/scale the frequency domain plot. Most of the time I'm using it, I only really care about the low frequency (vocal range) component.
As an aside, I find it a lot easier to bring in consulting gigs if I aggressively open source tools that I make, and then promote those OSS projects. I find it more pleasant to tell people about free tools than to just promote myself alone, but they end up feeding into each other. Just my two cents.