I don't think the cost of expanding to other regions/AZs is necessarily linear such that adding a zone would incur 50% more costs. Going from one zone to two wo

I don't think the cost of expanding to other regions/AZs is necessarily linear such that adding a zone would incur 50% more costs. Going from one zone to two would probably look that way (or even one server to two), but when you start going from two to three or even 10 to 11 then the %change-in-cost starts to decrease. This is even more true if/when you load balance between zones and aren't just using them as hot backups. As another commenter pointed out, Netflix says they have three zones and only need two to operate.