$10/month is one drink in the pub on one Friday night out of four. It's not even a movie ticket.
European electricity rates are closer to $0.30, and I agree that 100W 24/7 is a cost worth being aware of. I think we're seeing in this thread that it's pretty easy to find a system with standard PC parts from the past decade that idles in the 50W range, like OP, even with a couple of HDDs, and $50/year (US), even $150/year (EU), electricity cost to keep an old desktop out of a landfill maybe doesn't seem so bad.
I mean, one should think hard whether their home lab really needs a second full system running for failover, or whether they really need a separate desktop-based system just for NAS. And maybe don't convert your old gaming rig and its GPU to a home server. Or the quad-Xeon server that work is 'just giving away,' even if it would be cool to have a $50,000 computer running in the basement.