For reference: over the course of a year, if you have an uptime of 99%, that means you are down for 3 days 15 hours per year. If you have 99.9% uptime, that means the downtime is roughly 9 hours per year. If you have 99.99% up, then the down is ~ 53 minutes per year. 99.999% then gives you ~5 minutes of down per year. 99.9999% will then give you ~31 seconds of downtime per year.

One thing to keep in mind is that for each 9 of uptime you have, add about two Zeros to the budget and increase the timeline by one time units up and then double it. So if 99% costs you $100 and a day of timeline on the project, then 99.9% will cost you $10,000 and two weeks of timeline, and 99.99% will cost you $1,000,000 and 4 months of timeline, and 99.999% will cost you $100,000,000 and 8 years of timeline, etc.