Well it depends on the user. If it it a user like me, that has 1 album with 10 photos and update my status once in a blue moon... well not a lot, but, if it's somebody like my mum's cousin that updates there status every few hours with 10's of comments, and has around 30-50 albums with an average of 100 photos each, well it costs them much much more.
Until they go public, it's impossible to really know. Even a rough estimate could be way off.
Find out their monthly burn rate and divide by 600MM
So, in may 2008 they were said to burn 150MM / year.
Lets assume thats doubled - so, 300 MM per year.
300/12 = 25MM per month
25/600 = .04 cents per month, per user.