I have computers which have run 24/7/365 since 1999 (those 3 are the oldest ones) without any trouble ...
I have had disk failures, but those disks were more than 8 - 10 years old (usually 10 GBytes).
I have hade power-supply-units (PSU)-failures, but those were either very new or extremly old (more than 10 years)
The costs are mostly electrical power and your time. It takes quite a few hours every year to maintain the comps: cleaning the dust out, checking the capacitators, updating the software, cleaning out all useless stuff), keeping up with the science and the stats, doing some stats ... but that is the really fun part for me
