How much power does DDR3 RAM draw when unused?
You need to look into the Power Calculation technical note, which can be downloaded from Micron website. The EXCEL sheet mentioned by the hardware guy is also a good resource. His number around 100mW makes more sense to me. 

An easy approximation (only the background power) could be Idd2N * VDD. To get the detailed number, you need to add all dynamic power (activate, precharge, read, write) and also the termination power, which should not be ignored, especially when your DRAM is heavily utilized. The refresh power usually will not affect the average power too much.