Somewhere in the 5 digits. You've looked it over and can't find any holes in it. You congratulate yourself and head for the managers office. You had all of the bases covered. The problem is that you listened, but you did not hear.
High availability is extremely subjective.
There is a world of difference. If it goes offline, customers can't order. Amazon loses money for every second it is offline. Customers can get frustrated and go to a competitor that is simply a click away. Some of these customers might never return.