Both Google and Apple tried to have the $1 billion figure hidden from the public when it came to light in court documents for a case between Oracle and Google. Journalists got to it first though. You can read the whole story in The SEM Post here. Besides a billion dollars, Google also pays Apple "a revenue share of all the ads clicked from the Google search results - the figure is said to be 34% although it is not clear in whose favor that is."
Yowza! I guess they had to really sweeten the pot to make Apple interested though. It's not a deal unless both sides are profiting.