Other factors besides video card usage. Other computer components, fans, and air conditioning. If you air condition your home any energy that goes into the home is just as costly to extract from the home. In the winter it may make more sense but if you are using 200 watts as your metric, you are way off.
I was just trying to keep it simple. I have a rig of 4 5830's that pull 900 watts from the wall that's 225 per card (if you average in the watts used by mobo and cpu ect.) If you add a 5th or 6th card to the rig it goes very close to 200 watts each. So that part of what I was saying is correct.
When you say you need air conditioning that's not entirely true. You can throw your rigs in a basement and they will be perfectly fine with out ac and you wont be effected that much by the heat. If a basement is not an option then I'll point out that opening the window now is its own form of ac for the northern states. It gets to between 50-60 degrees at night which is as low as any ac will go. During the day it doesn't really get hotter then 80 degrees so you could just suck it up and not use ac because the computers wont be harmed it will just be a discomfort for your self. This discomfort will slowly go away though as it gets colder.
Any other factors I should take into account?
You're making so many horrible assumptions it's not even funny.
1. Not everyone has 4 cards in each computer. I have a couple of rigs that I am running with only 1 card in each of them, simply because that is all the motherboards will support (and I don't want to mess with trying to get a 1x slot working).
2. Not everyone has a basement. I don't. Actually, there's very few homes in Oregon that DO have a basement.
3. Yes, I could open the window, but I don't want to. I'd rather use A/C, because I get very uncomfortable in temperatures above 73F or so. I also have pretty horrible allergies depending on what plants/trees/grasses are letting off their pollen.
You're completely right that a person COULD mine 30 BTC for $200 in electricity if they had $0.15/kwh electric costs. But that doesn't mean that everyone mines that way. For me, it really DOES take $200 in electricity (or thereabouts) to mine 30 BTC.
So take your own advice:
Get your facts straight before you start spewing bullshit.