You can pay for piece of content i.e. standardize (approximately, of course) various types of content and set fixed prices for it.
This is popular model for paying for content (e.g. that's how Huffington Post/Engadget/TechCrunch are/were paying their editors for posts).
If you do that, the issue boils down to: does the price you can offer can attract content creators of satisfactory quality?
Your situation is unique but in general rule there are far more people out there capable of writing content for your app than there are apps they can write content for, so demand/supply is on your side and the examples I mentioned above tend to pay very little.
You mentioned that you want to limit pool of content creators to small pool of 4-5 but that might be a self-imposed limitation. In the model of paying per piece of content, your best strategy is to setup a reverse auction i.e. let people bid on the tasks you provide. The key for that is ability to clearly define what you expect to get as a final product.
This article http://www.kalzumeus.com/2010/07/17/seo-for-software-companies/ talks about how to outsource content creation (writing, in this example).
Paying a percentage of sales is a terrible idea. First, like you noticed, it's hard to determine the percentage.
Second, total percentage is fixed at 100%. The content presumably must be updated ad infinitum but once you give out all 100%, you have nothing left to offer.
Third, it provides anti-motivation (the income gets more tied to the overall success of the app than the effort of a single content creator which enables e.g. one bad apple to not do any work and profit indefinitely from your effort and efforts of other content creators).