How much does a "Euro Trip" Cost?

Next summer I plan on going to Europe and spending about a month there so for people who have experience going to Europe, I was wondering how much ESTIMATED it would cost. I know it depends on a number of things, but just a ball park estimate would be nice. I don't plan on living luxurious at all, mostly backpacking and staying at hostels. I want to start in Sweden and work my down to Portugal hitting about 8-9 different countries.