One thing I've learnt over and over with launching new apps is there are always unexpected hurdles that you'd never have thought about when you first started writing the software.
What are some you've run into, perhaps like IP for the songs in the app? Is it a difficult legal process to get a melody for a popular song into the app?
The biggest unexpected thing for me was that the simpler you make your app, the more likely people will enjoy it! People always make fun of how the Angry Birds game mechanic is so simple, but that's why it appeals to so many (kids and adults alike). Tiny Piano is dirt simple to play, and that's why users like it.
As developers, we like to always add features. Sometimes, removing features (and making an app simpler) will actually improve the experience.
Regarding IP for the melodies, it is not clear who to contact to arrange a license. Different parties own the copyrights to the lyrics, sheet music, performance, etc. It turns out that my app includes none of those. It just happens to have a collection of notes that, when you tap at the correct tempo/rhythm, sounds very much like a song you may have heard before. ;-)
But really, I try to be a good citizen and link people to the iTunes version of the song so that they can buy it and give money to the artists. With LinkShare, you can even get a 5% commission for each sale you generate. This comes out to about $40 a day of extra revenue for my app.
In the end, if the app becomes super successful (> 100 million DLs), I'm sure the appropriate parties will contact me, and I'll be happy to make a deal with them then!
The first commerical app I built myself was called Track My Ride, http://www.trackmyride.com.au. It earns me a fair slice of pie each month but nothing to retire on. Track My Ride was focused primarily on the local market that I could direct market to. My second app Rocket Mail Merge http://www.rocketmailmerge.com was actually born out of the need for easier invoice creation for my first business. It is targetted at the international market this time, primarily the US. I finished its coding at the end of last month. It's fully functional but I can't release it out of it's 'open beta' due to the abysmal lack of international credit card processing for Australian startups. It has taken so far 6 weeks to get approval from the bank, and that approval is still pending with no ETA in sight. I plan to write a topic on that in due course but not before getting my approval haha. Once I launch it I'll be writing a Show HN on it. It would be great if people found it as useful as I do.
You charge $30 for your products, so the scale of "success" will definitely be different. If you can convince 100 true fans to love your product, you'll make enough to survive, and iterate/improve.
When I shipped my first apps, charging $0.99, I had a really hard time convincing users to download. The whole first year, I made about $1000. (And I was happy!) I would have been ecstatic if I had ever reached 100K downloads @ 99 cents. In the space of Free Apps, the scale of success is different. I wouldn't be very happy getting 100K free downloads. :-)
Now I'm not saying I am not happy with 8 million DLs. It's pretty frickin' awesome, and I had never predicted it. But it's really a psychological trick to reset my bar at 10 million downloads, or 20 million, or even 100 million, to convince myself to keep working hard. :-D
Good luck with your bank approvals!
I just looked up your landing page for Tiny Piano. It's nice and informative but the grey background is a bit striking for the black iphone. The contrast would be better with a lighter color. You can pickup some really nice templates from themeforest for a few $ if you're not a designer at heart.
Best of luck cracking 10M downloads! I'll add an extra download to your totals shortly.