I don't think they are intended to be used as the only source of nutrition for 3 meals. I think you are meant to use two a day and eat a meal in the evening, a bit like slimfast? I think they have too many carbs in to keep you in ketosis too from what i remember. I don't think it would do you any harm for a week as a one off if you don't mind the risk that you might feel really awful but any longer than that and you risk making yourself really quite unwell so i wouldn't recommend it. Plus it will be extra hard if you can't get into ketosis.
It probably is actually cheaper to stick with exante as you don't have to get any other food? Although i know it's a pain having to come up with 100.00 at once it's probably worth doing it if you can.