Disclaimer: I work at Backblaze. We make the decision to switch to the more dense drives when the "break even" point is about 1 year of operation. We have a little spread sheet (it isn't rocket science) of how much electricity a drive uses, what we are paying for the physical space rental, etc. We plug in the prices if the 4 TB drives pay for the overhead within 1 year we go ahead and buy those.
The drives seem to last about 5 years in our experience, so technically we should be able to buy the more dense drive it it pays back in 4 years, but cash to run the business is very dear to our hearts so we don't like going out much more than a year on a payback.