Quick question about cost per day...

[Countzero]

My girlfriend and I are traveling for three months in Western Europe this summer. Starting around may 23rd in Amsterdam and working our way down France to go to Sonar music festival in Barcelona.

Then up through Austria to Berlin, fly to Rome and work our way through Italy and Greece.

I am wondering how much I should budget per day? Assuming we are trying to stay at the cheapest hostels/couch surf, buy our food & drink in groceries and generally try to make friends and go cheaply.

Is 60$ each a day going to be doable?

Also I am assuming that it is good to bring lightweight sleeping bags for camping possiblities but I am thinking my laptop should be left at home..
Any advice is very appreciated.

Thank you,
sasha