I guess expensive is relative but last I checked it was about $1,100 for a 300sq ft office (and they clearly have a lot more than 300sq ft). That was a few years ago when I had a friend who moved his offices out of there because the location wasn't worth the cost to him.
This is exactly the logic that our mteam used in the '99 bubble when we built out offices on 2nd. I'm not saying Dropbox is extravagant, but this isn't a compelling argument to me.
