Mailbox Reportedly Cost Dropbox $100 Million

That's a staggering sum for a just-launched product.

"Everyone is thrilled with the outcome," investor Saar Gur of Charles River Ventures told Business Insider earlier today.

But here's another perspective on the price: Dropbox was valued at $4 billion in its last private financing. It is probably worth more today—say $5 billion, based on Dropbox backer Paul Graham's rough estimate.

So $100 million is just 2 percent of Dropbox's total value. That doesn't seem like a huge price to pay to launch Dropbox into a new market and add 13 employees (including, by the way, Adam Cue, son of top Apple executive Eddy Cue).