What about charging monthly? If you could set a lower value than 20-50$ per month and your client life-time is something higher than 6-8 months, you can create something more interesting.
It's - as I think of it now - genuinely a desktop app and therefore somewhat difficult to set monthly price tags.
Spotify is an example, but you can find many others like Evernote, Steam or WoW. It depends on your users willing to pay on a monthly basis. But you are the one that best know your product, if you say it'd difficult, it must be true :)