You can change prices for in-app purchases. You can have annual subscriptions too. And promo-codes. What's the problem?
I am one of the few people who has the same app both in the (Mac) App Store and outside of the store. My issues with the store are all technical and sandbox-related. As far as payments and purchases go, the App Store is better than anything else by far. Don't be fooled by people who say you can do it for less than Apple's 30%. That's not true, at least, you can't do it legally. It is true that many people don't do business legally.
You may have some problem implementing your ‘promo code’ system into Apple’s IAP because they don’t want you to ‘sell’ a promo code and use that sale to get around IAP. For example, purchase the IAP for $10 or buy a promo code for $6 and get the same IAP for $2. So you can’t sell or otherwise get value for a promo code. If you tell App Review that you will not sell promo codes they may allow it. To use a promo code just create 2 identical IAP products with different prices. If the user enters the code offer them the lower priced IAP.