JonnyRotten,
I think one of us has miscalculated. If, according to your figures, 1/2 gallon of BL = 15 gallons, and it costs $100, then you are paying $6.67 per gallon.
If your homebrew pale ale costs $50 for a 3-5 gallon batch then you are paying $10-16.67 per gallon.
In this case it's...