Less than 50w idle power usage is achievable on an AMD system if you have a motherboard that allows undervolting. Lower the CPU NB and HT clocks, then lower the CPU and CPU-NB voltages. My Athlon x4 620 can function at 1.175v core (2.6Ghz) and CPU-NB at 1.025v (1.6GHz) without problems, enable all power saving features and you will see a lot of difference. My Athlon 170 also ran fine at 0.975v even if unlocked to dual core. If you dont run any cpu intensive tasks, you can lower the voltages a bit more. The power draw will also depend on what motherboard you are using. For FM2 processors, get a Gigabyte F2A85X-UP4 motherboard.

http://www.anandtech.com/show/6511/gigabyte-f2a85xup4-review-are-powirstages-needed-with-trinity/4