in order to get an average of how much you will need to backpack you will need more details for your trip. ex: mode of transportaion, how often you you use the transportation, how much you eat, where you plan to eat(home cooking, restraunt, cafe, fast food), drinking (beer/wine/howmuch), bars, walking or subway, sleeping(cheap or expensive hostel/hotel/parkbench/tent), flying(peak or low season), and so on.
Its impossible to say how much one person will need. I spent as little as 30 euros (40 dollars) a day for everything and some people can spend as much as 120 euros a day. I bought a consecutive rail pass for 1 month for 600 dollars. Others may buy a 3 month long rail passes for a couple grand.
Here is a link to a post from a couple of months ago that might be able to help some questions Basics
Welcome to the boards! You can never ask to many questions here