Probably not that much, less than a CRT, but if you're not using the monitor, switch the power off. I don't just mean standby, cos that can use 25-40% of normal power.
If you want an exact answer, then you will have to look at how many watts of power your monitor uses. Then contact your electricity provider and find out what rate they are charging you per kilowatt-hour, i.e. the cost if you use a 1Kw appliance for an hour. Then it's a simple calculation to determine the real cost of that monitor being left on!