60€ a month (tplink smart plug, calculated on the cost per kwh and other taxes). I try to pay most of it myself with money from side projects
60EUR a month is also pretty cheap for a hobby. If you can't count on your parents for even that level of support you might be better of getting emancipated.
The cost of running a PC 24/7 used to feel insignificant, but not so much any more. I've been running an old PC as a home server for years, but should probably find a more power-efficient NAS box to replace it with.