Mobile users don't expect to pay for apps, ever. Even if people are willing to pay, there isn't any recurring revenue unless you charge for upgrades and the upgrade would have to be to be significantly better than the version they already own. So the question is, how much could you improve a product like this, over the course of life time of the business?

Not to mention that mobile users generally feel entitled to free upgrades - TapBots are having this problem right now with TweetBot.