I'm surprised that people are making this argument here on HN. Pricing has nothing to do with how many months a developer spent on an app. The consumer doesn't and shouldn't care about it anymore than a developer should care about how much effort someone took to earn and spend those five bucks.

FWIW, I think five dollars is ok for a niche app like this but "he spent a lot of time on it" doesn't sound like a legitimate justification for it (or for any pricing).