Let us not forget that the housemates are also paid a fee for taking part...wasn't it about £14 per day a couple of years ago?
I do believe that the prize money should be kept, if only to keep the minds of the HM's focussed.
If you recall, it was the suddon realisation that the prize fund was rapidly disappearing, that caused the HM's to bundle Kitten out of the door during BB5.
With a bit of thought, I would suggest that the producers could come up with some interesting ways of exploiting the greed of the HM's. They tried to do this with Eugiene last year - but I think they could have made his decision much harder if they had told HIM that the fund would be doubled if he refused the offer.
I think it was also a little late in the game for any feelings of betrayal to develop.
One suggestion could be that a HM (probably a task winner) gets to choose whether to take £5000 in cash - but by doing so, puts the house on a very low food budget.
Lack of food will always cause tension...but at the end of the day money fuels more arguments in life than anything else.
Alan, Derby, UK.