I've bought over 100 apps, but it'd be quicker just to tell you what I never regretted, started from most recent to oldest, and what I paid:
The Walking Dead Season 1 Pass - $15
The Room Two - $3
Cerberus Anti Theft - $5
Rayman Fiesta Run - $3
DraStic Emulator - $8
Clash of Heroes - $3
Swype - $1
Poweramp - $1
The Room - $3
Rayman Jungle Run - $1
Nova Launcher Prime - $3
Titanium Backup - $7
World of Goo - $0.25
Two conditions to that list. #1 What I paid and what you'll pay today, tomorrow, or ever might be two different things. I bought Rayman for $3, while others probably picked it up for $1. #2 Whats now on that list, are titles I don't regret, and infact would have paid up to double, or even triple that amount if I had to buy it again in another life, with the exception of Walking Dead. There were other great games that I've bought that I do regret like Final Fantasy 6 for $15, or Deus Ex or Sega Racing for just $1. I regret 80% of what I've bought because I just don't play them, I've squandered my income on them. I've bought Assassin's Creed Pirates and Chaos Rings, and I've never even so much as installed them once in the past months since I've bought them. I've bought many an app, but the ones up above are ones I'd buy over again, because I don't regret them. I consider them Android Essentials.