Shouldn't be too hard to figure out. The power supplies for both computer and scanner should have specs on them that say how much power they draw.
Electricity is charged by the kilowatt-hour, which is using 1000 watts for one hour.
If you were drawing 100W, that would be 0.1 kilowatt-hour per hour, which gives you 2.4 kilowatt-hours per day. Then you would need to find the cost per kilowatt-hour on your electric bill.
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