What you've posted tells me this is the way your electricity is priced.
In one billing period (one month?), the first 160 kW-h is priced at $0.033 (3.3¢) per kW-h. Assuming you use more than that in the billing period, the next 140 kW-h are priced higher, at $0.072 (7.2 ¢) per kW-h. Then the next 200 kW-h are priced at $0.086 per kW-g. This is a common pricing system, used to provide an incentive to you to use less electricity, because the more you use, the more expensive it gets.
So, let's assume that in a billing period with NO use of your computer, you use between 600 and 750 kW-h. Now if you DO use the computer, the extra electricity will be costing you $0.158 (15.8¢) per kW-h.
Now, your original post says the CPU consumes 90 W...