You guys forgot to consider the cost of caps if you bottle or CO2 if you keg.
Okay I'll be the first to say it, since its inevitably going here:
What about the cost of your equipment? Electricity to run your refridgerator/ferm chiller? Your personal time that it takes to fix that leaky faucet or clean 100 bottles a week?
You have to draw the line somewhere.
Me personally I look at cost of ingredients and immediate operational costs that go with the brewday. Stuff like propane, electric, water, ice, etc are what I count in.
If you just look at ingredients I can be below 10 bucks per 5 gallons for a simple pale ale or the like (reuse yeast, buy hops and grains in bulk). But when you add the operational costs I estimate that goes up to between 15 and 20 dollars.
Propane is expensive. I've started to battle that cost by building a E-HLT, now I can get between 3-4 10 gallon brews off one tank. That puts the propane cost per batch at around 2.50 per 5 gallons.
Ice is also expensive. I buy 10-20 lbs per brewday usually. This is like 3-4 bucks. I have fought this cost by freezing 2 liter bottles a few days before brewday.
Water is not so expensive since I have a dedicated filter at my brew area. Sometimes I will use bottled spring water if I feel like it. I havent noticed much of a difference so I guess my tap water is pretty good.