I think the problem is a lot of folks probably dont remember how much their bills were before aquariums, or its been so long that prices have changed, etc. etc.
Its been about 4yrs for me that I've been in the hobby. I do remember what the electric was before that since it was a new house and I was impressed with the efficiency of the heat pump, etc. However over those few years other things have changed, like I usually have 3 computers running 24x7 now that I didnt before, big screen tv, etc.. My bill now-a-days is about 50% more than it was 5 yrs ago. Also monthly usage can fluctuate a lot, to get a good comparison youd have to average it over an entire year to account for varied usage due to unusually warm or cold months, for example (especially for those with electric heat/air conditioning).
Also dont forget about water bills. In some municipalities this doesnt add up to much, and obviously it depends on how many gallons a week your going through. Mine is on the pricey side, and its the sewage that really gets you as they basically just take your water and double it.
I think those Kill-a-watt things would be about as accurate a way to measure the cost of the aquarium for most folks.