Seriously. Whatever number you're thinking of, it is too low. Think of a number that makes you wince. That number is too low, too.
Many people here will suggest you charge like $5 or $10. These people are unwilling to buy your application at any price. They are not your customers. Their opinion on your price is irrelevant. (I mean this in the nicest possible way, guys.)
Charge for value. It isn't a "little" app or a "simple" app. Your customers are not programmers and do not know how many LOC it was (or, on the other extreme, how much loving care you put into it). They only see the value delivered to them. Price appropriate to the value.
People pay more money than you will ask, far more, for things which matter far less to them. Always remember that!
Charge more.