If you are selling your ebook through the Apple's iBookstore, Apple retains 30% of the sales price after any applicable taxes have been deducted.
Note that if your ebook sells through one of Apple's international iBooks Stores, Apple may price it differently (lower) according to that market. For example, your ebook may be sold for 99 Euro cents instead of 1.99 USD (or whatever it was set to), or 4.99 EUR instead of 9.99 USD, etc. This means your profit may be lower than if it had sold via the US iBooks Store or the Blurb bookstore.