The 15 day Best of New Zealand Seat in Coach/Self Drive package only includes daily cooked breakfasts at hotels. All lunches, dinners, and snacks are on your own arrangement. And it is really up to you how much you want to spend and what you choose to have.
To give you an idea, A cup of good coffee costs NZD4-5, a sandwich costs NZD4-5, a light lunch at café or fast food restaurant (eg. KFC, MacDonald, Burger King, Subway, etc.) costs around NZD10, a humble dinner at a small restaurant could cost NZD15-30 per person, but a nice 5 course dinner at a fine dining restaurant could cost NZD80 per person without counting in the drinks.