Not a no-brainer for sure, but depending on what PC it is, the costs can still be really low. Assuming 50 W power draw (you can get this much lower with cheap or free e-waste PC's like old Atom NUCs or thin clients, or a low-power laptop like an old MacBook Air) you're using 1.2 kWh per day. Even at current extreme electricity prices this will not really break the bank for most people.

Better, forget about running 24/7 and use rtcwake(8) to wake it up for an hour at night to perform the job and then go back to sleep. This will bring costs far into the negligible.