They do prohibit developers from breaking down the costs like that. This is the guideline they would cite you for:
> 3.1.3 The following apps may use purchase methods other than in-app purchase. Apps in this section cannot, within the app, encourage users to use a purchasing method other than in-app purchase.
Yes, it should be illegal for them to prevent you from breaking the cost down, but they have historically interpreted this statement that way.