Seems like a pretty large number, but remember, funding isn't dropbox's only source of income.
In addition, depending on the definition of 'marketing' (IANAL), the referral engine (i.e. free space for referring other users), might count as marketing the dropbox brand, and the 'value' of the free space would likely count as the amount customers would have to pay, not the cost to dropbox.
In addition, depending on the definition of 'marketing' (IANAL), the referral engine (i.e. free space for referring other users), might count as marketing the dropbox brand, and the 'value' of the free space would likely count as the amount customers would have to pay, not the cost to dropbox.