My absolute first advice would be not to launch the app at a low price then raise it up later on. That's an absolute No No.

There is no real rule to price your app. My suggestion would be to look around the app store of similar apps to get an idea of what they are priced at.

Once you launched the app at a certain price you can always have limited time deals. Keep an eye on the downloads and price. By doing this you may be able to find the perfect price that gives you the most sales and can change the price accordingly to that.