$4.99 Garageband.
but I *WOULD* gladly pay MUCH more than that for many apps if they had a trial try before you buy preview app. Text descriptions and reviews and a handful of screenshots simply isn't a sufficient way to tell if the app does what you want. I downloaded a $2.99 game on the good reviews it got and the good screenshots, but it turned out the game was super lame. It was just 3 bucks, but I felt burned. After that happens a couple of times I'm TOTALLY turned off to apps that don't have a limited preview lite version available.
When I use a light version of an app I'm literally almost infinitely more likely to buy it, because I basically never buy apps that don't have a limited trial version to try out first. And for apps that I do try out with a light version, I usually go on to buy the full version probably around 60% of the time. That's pretty damn good.
TL;DR let me try out a lite version of the app and I'll usually GLADLY pay a significant amount of money if your app does what I want.