P3 Kill-A-Watt P4460 Electricity Usage Monitor for $32.14 shipped.  Kill-A-Watt is a tool that helps identify power-hungry devices by measuring their actual usage.  This is a newer version that includes cost calculation in addition to power usage and other related measurements.  We have a couple of them and have found them to be, uh, fun, amusing, and more importantly, very useful.  Here is a review for your reference.  Most places have it for about $40.