You know, this could've been a very simple way for the producers to get rid of all the racists(or "mean ones", if you're Dermot O'Leary).
They could've just got them all in the diary room, said you got half an hour to sort out this shopping list, your final total must be the exact value of your shopping budget, not a single penny under or over. If you fail the task, you will be up for eviction. Since they're all morons, they wouldn't be able to figure out those complex numbers. And just to be on the safe side, the list could engineered so it's impossible to come to a round figure.