For the next 6 months electricity costs will be more or less irrelevant. I have free power too but note this:
1) The video cards will lose more in resale value than they will be able to mine per day
2) People with high energy use Avalon rigs don't even bother looking at the power use since they have BTC coming in like crazy. People with ASIC units will therefor not want to relocate their precious machines at the expense of unit downtime.
3) In 6 months when 66GH delivers 1BTC or less per day it may be reasonable to find cheaper power.
I still have about 40 cards left (had almost 200) and will be listing the 6870/6950s before they become near impossible to sell. Selling a 6870 for $80 and buying 0.8 BTC is a lot easier than having it mine for 7 months (this will be the case after Avalon Batch 2 and ASICMiner wave 2).
Have you looked in to litecoin mining? It seems like at the current difficulty your 6870 could mine 100 litecoins per month so with the 40 cards you have you could theoretically mine 4k of litecoins (Obviously unless you have free electricity your profit won't be 4k) .
EDIT: LITECOIN IS CURRENTLY AT $4/LTC