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

I pay $10/mo for the storage bump. Not sure how many there are like me, but I suspect a non-trivial amount of their revenue comes from non-enterprise customers.

I think it takes courage, because people will only pay if you provide some real value for them. Give out things for free and you might build a nice user base even if the offering is not so interesting.