An average hard drive uses 7 to 10 watts running full out - which in a mining rig, it shouldn't be doing. It uses much less when idle.
Worst case monthly cost for your hard disk at your cited energy costs is $1.44 per month.
A single 5830, on the other hand, would cost about $15.55 and one of my 400W rigs would eat up $57.6 per month. I've got six of those rigs and energy here in NV costs 11.28 cents per kW/H. My bill, including air conditioning, went up almost $300 per month. I envy your paltry $200