Ask HN: when do SSDs become cost-effective for home media server?s

I just replaced a 1.5 TB disc in my home RAID1. It cost me about hundred dollars. At what point should I consider changing out to SSDs? Presumably the chassis and motherboard, Which have been running since 2003, are okay indefinitely, Except the whole point is power savings. I guess there are power questions from the CPU, A 3 GHz Pentium something, and the disks. Possibly also the power supply if it is spending too much on conversion.