A good "rule of thumb" to remember: if your electricity rates average (somewhere near) $0.11/kWh you can take the average power draw of a device in watts and that is equal to what it will cost to run that device 24-7 for 365 days.

So, if that cheap PC draws 50W more than an alternate solution, it's costing you $50 more per year to use it.

Some tasks are beyond any RasPi, but it's well worth evaluating if something like an N100 fanless mini-PC can handle it instead of loading up some Core i7 rig that's going to cost more to run in the first year than the N100 costs to buy.