Software reliability is an important point. My personal site running ACS on a single sun x1 with no backup has better uptime that the site mentioned above. There are several reasons for this but mostly it does less and gets upgraded less.
However, perhaps the important (and overlooked) piece of high availability is how long to recover from a catastrophic failure. My goal was max 4 hours. The longer you have the cheaper things get. If you don't need a 2nd data center you can cut the cost in half. Unfortunately without one it's difficult to calculate max downtime.