Here are some comparison standards:
http://www.irs.gov/businesses/small/...104627,00.html
$400/month for a family of 4 doesn't seem out of line to me. I often enough spend that for just myself (not being frugal; food is my biggest challenge category).
I'll come in to the challenge in some form--maybe like you, buying the prepaid gift cards. What I know is that it would be really good for me to limit my spending to 25 more on gas and 165 more on food/household supplies for the month, and no other spending. (Well, dinner at Musikfest one night, but I have the cash for that in my wallet).