How much money (AUS or EUR) do I need for a journey with a friend through Europe?

My and my cousin are going on a backpacking trip through Europe, from Norway down to Italy, and I want to know how much Money we will need, each. We are planning to go just before Christmas, around Nov 20th - Dec 20th. I will be 16, and she will be 21, we are both female. We are also planning on using Eurail, and staying in hostels