[sangnoir]

The question then is, would it be cheaper to replace that one drive or get the more expensive disks with lower chances of failing?

[Arn_Thor]

Isn't there a cost to time and convenience too? Buying a new disk takes time, as does rebuilding the RAID. And during that time you are vulnerable to another drive failure which could be disastrous if you only have one drive redundancy, especially during the very intensive rebuilding process.