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.
It's more like going from the stone age to the bronze age. You need expertise and architecture suitable for achieving high availability, these are your budget increases, they are not linear at all. But the infrastructure itself doesn't necessarily get more expensive, on the contrary, new architecture could allow you to use the cheapest stuff available on the market.