Rule of thumb is that cost scales as the square of the number of nines in the percentage uptime. If 90% uptime is OK, 99% costs 4 times as much, 99.9% costs 9 times as much etc. Reliability isn't all that cheap, but we aren't asking for the sort of five-9's reliability that Amazon look for. 99.9% is only down an hour a year after all
(edit: I seem to be making more posts about computers than flying at the moment. Bl**dy weather...
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