Very low. I estimate the power consumption at the wall to be ~400W. That's $29/month at $0.10/kWh. Host this in a datacenter with a non-remarkable PUE of 1.5, and that's $43.5/month including cooling.
* 100 Watt for the mobo, CPU (73W TDP), SATA controllers, and all the fans. Source: my own clamp-meter measurements on dozens of PCs http://blog.zorinaq.com/?e=42
* 45 x Hitachi HDS5C3030ALA630 drives, 4.8W each (average when idle), a tiny bit more when seeking so let's round to 5W. Source: http://www.hitachigst.com/tech/techlib.nsf/techdocs/02D91977... and my measurements that HDDs seeking only consume a tiny bit more current than when idle http://blog.zorinaq.com/?e=50
* 100 Watt for the mobo, CPU (73W TDP), SATA controllers, and all the fans. Source: my own clamp-meter measurements on dozens of PCs http://blog.zorinaq.com/?e=42
* 85% average efficiency of the Zippy PSM-5760 power supply. Source: its 80PLUS certification report http://www.plugloadsolutions.com/psu_reports/SP105_EMACS_HU2...
* Hence: (5.0*45+100)/.85 = 382 Watt