You should e-mail Apple developer support. I'm almost positive what you want to do circumvents their policy; they want direct control over all transactions that result in the user getting an app.
I have a free app that lets you upgrade to the full version with a 99 cent in-app purchase. I don't remember ever seeing the option to do it for less than 99 cents. Like nadavhart says, pricing works in tiers.
Good morning! I'm a long-time software developer, but still relatively new to mobile development. Just here to see what others are playing and working on, get settled in and make some new friends.
Cheers!