If you run it 24/7 on a dedicated desktop with decent idle (i.e. no high power video card, low power CPU), it likely uses ~ 50W average. That gives an estimate of 0.05kW24hr30 = 36kWh, which would be in the range of $12/month at current australian electricity prices. If you have bad idle power usage (or somehow mostly active not idle), maybe you'd be looking at twice that. But for OP who was spending $xxx/month on subscriptions, it's a pretty negligible cost. If you really want to save this too, raspPi can do a lot of home server needs nowadays
My optiplex shows as 8.191 kwh last month -> 11w average.
OP was trying to replace streaming services, so he gonna need storage. Assuimg he had 3 hdd for raid 5, that is another 15*3 = 45w
There are also noise and light pollution issues if it's near a bedroom.