The best (most accurate) way for me to break it down is the cost of what I drink by ounce.
An average 6-pack of craft beer in SoCal costs me about $10. Sometimes less, sometimes more, but $10 after tax/CRV is a good average. This equals:
6bottles*12oz = 72oz = $0.138/oz
I also frequently buy 22oz bombers of beer that I can't find in a 6-pack. A good average for a bomber is $5 after tax/CRV. This equals:
1bottle*22oz = 22oz = $0.227/oz
Additionally, I'm not one to shy away from buying the 750ml Belgian, and these are considerably more expensive. A good average is $10/bottle. This equals:
1bottle*25.36oz = 25.36oz = $0.394/oz
A weighted average to compensate for the type of beer that I drink most often would be weighted pretty consistently at 65/25/10. This equals:
((.138*.65)+(.227*.25)+(.394*.1))/3 = $0.185/oz
So, IF I purchased a 5gal batch at my normal average price of purchasing craft beer, I would be spending:
128oz/gal * 5gal = 640oz*$0.185/oz = $118.4/batch !!!!!!!!!!!!
Therefore, as long as I'm spending LESS than $118.4 on ingredients for a 5gal batch, I'm saving money!
TO JUSTIFY TO SWMBO:
At my present rate of drinking an average of 40beers/mo. and if I purchased every single ounce, this equals:
40beers*12oz = 480oz * $0.185/oz = $88.80/mo on purchased beer
So, If I brew a batch, say even an expensive IIPA batch, at $50, this equals:
$50/640oz = $0.078/oz and then 480oz*$0.078/oz = $37.5/month
BUT HONEY, I'M ACTUALLY SAVING US $88.8-$37.5= $51.3/MONTH!!!!