I think you may be crunching the numbers the wrong way...
Dropbox is valued at $10b (regardless of what you think of that number, someone was willing to buy at that price). But they only have 1800 employees. If by some miracle 80% of them are engineers that is $7M per engineer. All this for a "pure software" business. No inventory, no manufacturing, they only recently started doing their own operations.
Investors would be telling them "more engineers" => "more software" => "better Dropbox". So now you have to go out and snap up engineers in Silicon Valley, which is very very hard. But you don't offer the perk of a Google/Apple line on a resume nor the compensation of e.g. Microsoft. What do you do?
Answer: you offer people the chance to work on cutting edge breakthrough technology. Not only that but it's all open source! This is a dream job for some people. They will turn down every other gig for this one.
It doesn't matter that PB/mo is chump change because an investor doesn't know that, they only know the engineering headcount. If by some miracle an investor does know this is a waste of time, then you just point to this HN post and observe how many developers are interested in this technology and how it is attracting developer mindshare that can be exploited for additional hires down the road.
I'm not saying it's rational–it's not. But I think there were strong incentives to greenlight a project like this, even if the actual cost savings were zero.