If the contract expires at the beginning of 2014, then there will be one year of sales, or around 250 million units (assuming some growth).
It could also expires some time during 2014, which is up to 2 years of sales. In that case, 500 million is a good estimate.
Also, this includes iPad processors, which cost a bit more.
With all of those assumptions, it's likely that the difference in cost will be somewhere around $1 billion. It could be more or less depending on sales and when exactly the contract expires.