That number is actually much more flexible: Facebook paid $300 million in cash and 23 million shares for Instagram. That means the amount Facebook pays for Instagram will fluctuate depending on Facebook's share price when it goes public.
That's assuming the deal goes through. Facebook also noted there is a $200 million break-up fee if it doesn't. Instagram will make money either way.
In April 2012, we entered into an agreement to acquire Instagram, Inc., which has built a mobile phone-based photo-sharing service, for approximately 23 million shares of our common stock and $300 million in cash. Following the closing of this acquisition, we plan to maintain Instagram's products as independent mobile applications to enhance our photos product offerings and to enable users to increase their levels of mobile engagement and photo sharing. This acquisition is subject to customary closing conditions, including the expiration or early termination of all applicable waiting periods under the Hart-Scott-Rodino Antitrust Improvement Act of 1976, as amended (HSR), and is currently expected to close in the second quarter of 2012. We have agreed to pay Instagram a $200 million termination fee if governmental authorities permanently enjoin or otherwise prevent the completion of the merger or if either party terminates the agreement after December 10, 2012.
For reference, here are all the rumors related to the Instagram deal I'm talking about:
Facebook is expecting to close the Instagram acquisition this quarter. If Facebook ends up going public at more than $75 billion, it will pay more than $1 billion for Instagram, which is what the price tag was previously quoted at.