I spent $18.99 on Starmap Pro. There were only a couple features missing from the ~$2.99 version, but they were deal breakers, so I guess I'm overall happy that I got what I wanted. Finally I had a planetarium app in my pocket again after putting my Palm TX to rest two years prior.
Close Second:
I just spent $14.99 on techBASIC (a BASIC program compiler, with access to iOS objects) thinking I would play with it on vacation last month. Still haven't touched it, 🙄 but do not regret the purchase, yet.
Back in the days of Palm Pilots, I made great use out of PocketC (a pared-down C-compiler) over the years. techBASIC is the closest thing I've found to that functionality on an iDevice.
Regards,
Tom