Creating an affiliate program for my iPhone app. Which action should I consider a conversion?

I have a free iPhone app with in-app purchases. I'd like to create an affiliate program for it to get distribution, but not sure which variable to link as a completed action to pay affiliates.

Should it be just on app downloads or should it be on in-app purchases? There are very few people who use the in-app purchases so I'm not sure if that will turn affiliates away.

If you end up paying an affiliate (something) for each download (free), then you're spending money to gain users.

If those users continue to not spend by IAP, then you're going to have a business model that costs more to run than it makes. This is okay, if you see that as an investment for "later" - although, you have no control over the quality of users being sent your way.

If you offer the affiliate a percentage of the IAP, then your business model leans the other way - you only pay out after you've made money - but, that's not fair on the affiliate. They can't control whether the new user is going to purchase, nor can they control what you offer for sale. I suspect you'd have to make the percentage very large to make this interesting for affiliates.

Deciding which largely depends on what you're trying to achieve in the longer term.