A rule of thumb might be at least a factor of two to three for each additional nine.
For instance, first you start with internal redundancy (RAID, redundant power) which can easily double your cost.
Then you have redundant systems, at least twice as many, which may more than double your cost. Software licenses and support contracts may need to be upgraded. A more sophisticated configuration will require training or hiring better qualified ops people.
Then you need redundant data-centers, which is obviously at least twice the cost.
Beyond that you will want to rack up redundant provider feeds, layer on more monitoring systems, have more ops people on standby in case of trouble, and so on.
Each nine is like a whole new world.
Nearly anyone can do 90% uptime. 99% isn't that hard. 99.9% is when things require a more disciplined approach.
For instance, first you start with internal redundancy (RAID, redundant power) which can easily double your cost.
Then you have redundant systems, at least twice as many, which may more than double your cost. Software licenses and support contracts may need to be upgraded. A more sophisticated configuration will require training or hiring better qualified ops people.
Then you need redundant data-centers, which is obviously at least twice the cost.
Beyond that you will want to rack up redundant provider feeds, layer on more monitoring systems, have more ops people on standby in case of trouble, and so on.
Each nine is like a whole new world.
Nearly anyone can do 90% uptime. 99% isn't that hard. 99.9% is when things require a more disciplined approach.