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. But that is not all of your computer. It is more likely that the total power consumption of your machine is 200 to 400W most of the time, but could be higher for heavy tasks. Let's assume a modest 300W average. So for every hour you use the computer at that power consumption, you would be paying for 0.3 kW-h at 15.8¢, or 4.7¢. If you do this for 3 hours per day for every day of a month, that might come to $4.27.