For example BackBlaze sells storage at $0.005/GB [1]. I picked them as an example, because I'm fairly confident their business model is based on making money with this. Based on this I would assume that at Dropbox scale you could do storage at least for the same price, maybe even cheaper. With those prices the free user storage would cost like max 1 cent/month. There's of course some users with more than 2GB, but there's also many who use less and all the de-duplication and shared folders pushing down the per free-user cost.
[1] https://www.backblaze.com/b2/cloud-storage-pricing.html