I also just reread their figures, wow! I spend $200-$300/month on groceries for my husband and I, which at the high point is $10/day and trust me, my husband and I aren't eating 500 calories each. I just posted somewhere else that my husband eats 3000-4000 calories/day and I eat 1500-2500 (low/high days).
If I take my low day and my husbands low day and take the high figure for a months worth of groceries, I get $2.2 per 1000 calories and their figure was $1.76 for 1000 calories. so my estimate is slightly higher but again that is a worst case and I have to say that I've had more 2500 calorie days in the past 6 months than 1500 calorie days (ugh).
And actually a more accurate figure would go for the middle so if I say I ate 2000 calories/day and my husband ate 3500 and we spent $250 in groceries, that would be $1.51 per 1000 calories which I think is a fair estimate.
You can't out-exercise poor eating habits.