Google incurred that cost early, but that tipping point of being 'worth it' to engineer for uptime came pretty late in the game for twitter.

Interestingly AWS appears to be in the camp where it's not heavily focused on uptime either.

A pretty universal truth is that you can afford to be down more than you expect. Even in the cases of Amazon's store being down when you can measure the immense cost by the second, a total cost analysis may still well show that it was a 'cheaper' event than engineering away.