How can i get cutomer proceedings on in app purchase completition via code.

I have PJSIP based voip app. I want to integrate in-app purchases to let user top up balance. My requirement is that i top up user with balance equal to customer proceedings. Like lets say if user pay 5$ he will get around 2.75 $ so user balance should be top upped with 2.75 $. Now problem is that this customer proceeding is different for different itunes store. If user is buying from European store these proceedings will be different.

How in my app code i can get customer proceedings so at our server back-end we can top up user balance with same amount.

Second thing is that is there any way to get this info before posting in-app purchase. It will be good to show user before purchasing that he will get hti smuch balance on spending this much amount on itunes.

I am willing to set it for Tier 3.