The clues are there; poor diet + lack of exercise = short life.
Whilst I understand what you are getting at; it is not always the case.
EDIT
Snack:
Apple 25p, mars bar 55p
Dinner:
Microwave spag bol £2.50 (35g of fat!), or any takeaway is goin to be £6+ and many times worse than that.
Home made spag bol
200g mince (so at least double the ready meal, ~10% fat) £5/kg, £1
Value Pasta 30p/packet ~ 7p portion
Tin of tomatoes 33p
assorted herbs, glug of red wine, salt/stock, chilli – 30p?
total = £1.70
How much cheeper does it need to be?
^^^ this – unhealthy food is less costly in effort not in monitory terms.