I don't really see this as a niche app. A niche app to me is an app that a particular segment of the population would be actively searching for. For example, runners would be searching for a "run timing" app. I think your app falls into the mainstream simply because people (I don't think) are going to be actively searching for it. And when they do find it on the new releases list or some other method, there is no instant recognition as to what it does, so your price will drastically affect your conversion rate. That said, 3.99 I think is way too high. I think 99c may even be too high. I would start off free with ads with in-app-purchase option to remove them.