Maybe it would be good to write the custom an email explaining stuff like
99%=Well run server
99.9%=Multiple backups, will cover most hardware failures
99.99%=Top grade commercial
99.999%=What companies like Google or Yahoo can achieve
99.9999%=Hopefully the US strategic defense systems are this reliable
Also, you're assumption about the servers are entirely independant. That's a reasonable assumption in terms of fires and blackouts and floods, but not for software problems. You really can't assume that unless the servers are all running entirely different software stacks on different operating systems.
99.999%=What companies like Google or Yahoo can achieve
Even worse. Five nines is five minutes of downtime per year. The core Google search experience blew five nines for half a decade with just one outage -- the one where they marked the entire Internet as a malware site, which took somewhere like 40 minutes to address.
This kind of thing makes me dismiss talk of nines as fetishism or sales-speak. You can say your system is going to have five nines of uptime at the application level. You're probably lying.
P.S. Pricing-wise, a client who wants > 99.5% either wants to pay mid-six figures (and up up up) or they want something which is deeply irrational for you to offer.
If we're talking about the agreement, SLAs that are better than 99.9% are quite common, and available even on low end products. The problem is that the SLA payout is usually "we refund you for the time you were down, if you ask for the refund" - heck, with that payout, I'd be happy to give you a 100% SLA on any product I sell. (of course, I'm not going to advertise as such; the sort of people who buy from me would find that disingenuous.)
That said, I think you are about right with 99.5% being about the best you can expect while spending a reasonable amount of money. (especially for a static site, i think a few more tenths of a point is possible for less money than you think, but the cost curve goes parabolic sometime after 99.5%)
99.999% is the standard for landline telephones, which I think is a pretty decent analogy here. Unless the customer has some crummy VoIP solution and doesn't trust their phones anymore :P.
I believe it's what they design for at the central switch nodes. The buildings would have entire rooms if not floors dedicated to nothing but 48 volt wet-cell batteries.
Of course the "last mile" infrastructure is not so reliable. That said, in over 20 years I've had POTS service, I can't ever remember not having dial tone when I lifted the handset.
