around $400m in annualized revenue as of last summer (http://www.bloomberg.com/news/articles/2015-06-24/dropbox-is...). no recent news of their growth, but I would be surprised if they did more than 20-30% user growth since then (not bad, but not a rocketship anymore...which FTR is totally cool with me, even admirable at this stage!)
Jives with back envelope calcs: 400m+ users, assume 1% paid users = 4m+ paid users. each user pays something around $100/year (8.99/month if you pay up front, 9.99/month if you bill monthly, some legacy cheaper, some business customers). 4m x $100 = $400m. It's actually not a bad business! =)
Not sure about costs, but they are cash flow positive from what they've said publicly (http://www.recode.net/2016/6/14/11937132/dropbox-cash-flow-p...) and that's been confirmed from what I've heard anecdotally/personally from some senior folks over there and some VCs who I think would/should know (and that I trust..ha).
so overall, seems like a solid business, i think the issue now is whether they can grow at a rate that's exciting enough to the market to justify a $10b+ valuation...TBD!
as one kind of comparison (not definitive by any means, just one that we have the #s to do..): box did around 300m in revenue last year (so less than DB), but Box market cap was around 1.5B at that point, so 5X revenue. if you did the same math to DB, we're looking at around 2B market cap for DB...1/5 of their last raise at 10B =/
could be viewed as example of raising at higher valuation than prudent (not unusual ha), but who knows, maybe they'll pull a rabbit out of a hat =)