Sunny is right, your explanation is a bit dubious in that it leaves much to speculation. For example, where do you want to add this extra clause, outside or inside the current IF call? Is it really 0 you want to test for, or NULL also?
See if this is a correct interpretation:
IF ([p-code] = [p-original], [sell-price-new], [sell-price-new] / [p-master-pack]) +
IF ([p-master-pack] <> 0, [split], 0) AS Contract_Selling_Price,
Note that in a single column expression you can only manipulate the value of the current field, not two fields at the same time.