This will vary depending on the computer in question and your local energy cost.
Here is a table listing power consumption of a range of PCs.
So, for example, let's take the worst case shown on the table. The IBM ThinkCentre M52 running Folding@Home with the monitor on pulled down an average of around 175 watts.
175 watts * 24 hours * 31 days = 130.2 kWh
Assuming a local energy cost of $0.15 / kWh would mean that it costs $19.53 per month to run a relatively inefficient computer at 80% CPU usage with the monitor on for 24 hours a day all month.
On the other hand, if the computer is drawing 1 watt while in the "off" state...
1 watt * 24 hours * 31 days = .7 kWh
At $0.15 / kWh, it will cost you approximately $0.11 / month to have your computer off (but still plugged in to the wall).
Of course, you will likely not find your specific setup in a table of power values (and even if you did, you cannot be sure that it will match your specific setup). If you want to find out the real numbers for your setup, you can purchase a power usage meter (e.g. Kill-a-watt). That info, along with your cost per kWh, will give you what you are looking for.
If you are looking for just a generalized answer, then my speculation would be:
- Leave my computer in standby mode. - Moderate
- Leave my computer in hibernate mode. - Very little
- Turn off my computer but the cord remains unplugged. - Very little