Hi!
There are few unpredictable expenses on a day-to-day basis, as you're really just eating, drinking, sleeping and walking. I've always found other pilgrims to be equally keen to eat at reasonably priced places etc, so it's unlikely your Camino family will all decide to go for a slap up lobster dinner.
It's useful to have access to extra funds for ease of mind, just in case your boots/backpack failed or some such catastrophe. It's also useful to have the option of staying in a hostal occasionally if places in albergues are short. Sharing a private room with others usually reduces the cost considerably.
The other consideration is the flight home, which is a bit of a trade off between flexibility and cost. You can book your flight home well in advance to reduce cost, but you may then feel you're walking to a schedule.
Buen Camino!