[nphase]

You have to debit the cost of Heroku's downtime from the cost of building your own infrastructure.

That's silly, and also not how it works, at all. You're paying PaaS/IaaS companies so that it's their headache, not yours. Once it becomes your headache, they are no longer doing their job, and you are no longer receiving value for which you are paying for. You don't debit the cost of their downtime from the cost i would've built your infrastructure, you debit the cost of their downtime from your business' revenue and reputation.

Whether or not you could do it better yourself does not excuse the downtime one bit.

[jimbokun]

Excusing or not excusing are irrelevant.

If you stop using Heroku and manage your own infrastructure, you need to take all relevant costs into account.

Of course, finding an alternative provider of the same (or similar) services is also an option.

[jemfinch]

Isn't it commonly recognized that it's cheaper to run your own hardware than to pay a cloud provider? It just requires more capital outlay and maintenance.