Re: Sounds good!
I'd say the numbers completely make sense over doing it yourself for all the reasons listed by MatthewSt.
In addition you have the fact that these prices tend to decrease year on year. You don't pay for anything you aren't using yet with cloud storage so for a typical use case where you've got an archive that will build up over 5 years and only max out at the end of the 5 years you aren't paying that maximum until the end of the period where the price will have more than halved anyway.
Unfortunately typical in-house projects will demand you pay for the storage array requirements for at least the next 3 years in advance, at today's prices.
Bear in mind this is just for archiving requirements, not Tier1 storage for a DB. The economics of it just make sense provided it matches your requirements.
Biting the hand that feeds IT
