Think about what shady gossip newspapers pay paparazzi for breaking laws to get intimate pictures. In the right hands this exploit could be worth tons.
But that assumes the hacker is going to be his own fence. An arms dealer doesn't sell a gun on the basis of "yeah but if you jack 10 nice cars you'll make over $25K, so the gun is at least worth $4K".
Nonetheless, it just _feels_ that $4500 is a small amount, coming from such a large company. What is the downside of FB offering a higher reward, like, say, $20K? Or would the same argument apply, $20K is nothing compared to the millions tabloids would pay for a photo?
It's also not the case that this vulnerability == celebrity photos.