The "official Instapaper app" designation does come with one significant carrot: users of your app wouldn't have to pay the subscription fee that others have to

The "official Instapaper app" designation does come with one significant carrot: users of your app wouldn't have to pay the subscription fee that others have to pay in order to access the Instapaper API. Is that enough to justify a 50/50 split? I don't know, but it is more than a blessing and promotion.