We just read the rules (why didn't we do that before we agreed to sign up?) and you have to budget £5 for what goes in your shopping cart to last you five days. You can't buy a bag of rice and say 'I'll only use half' and count it as half the price. Although they do recommend buddying up with someone so that your shopping goes further (eg you and a friend could buy the rice together and pay half each).
Matt and I have £10 to share and we have to factor in WHOLE cartons of anything we've used so far. I suddenly resent the fact that he used peanut butter that has cost us £1.67 and I don't even eat it!!
Luckily I'm a very forgiving person. I probably won't hold it against him - for too long.
Working out what we've used so far means that we have spent:
Peanut butter £1.67
Bread 2 loaves £1.00
Cup of soups £1.00
We've got £2.00 left.
I want to spend £1.60 of it on 350g of tofu. Matt might not be keen, but then he did use peanut butter...