This is a very difficult question to answer as spending varies dramatically from person to person.
As a VERY ROUGH guide we say allow €60 (Euros) per day:
Accommodation usually varies between €15 to €25 per person per night sharing. Private rooms will be more expensive, probably in the €25 to €35 range (per person).
Allow €20 to €25 per day for food, but remember that you don't have a fridge, so you're either eating baguettes and cheese, or eating at a local restaurant. Some hostels/campsites have self-catering kitchens, so you could pick up stocks at the local supermarket, perhaps splitting the cost between a few of you and cooking up a big meal.
If you're eating out at restaurants (and it's a great idea to try out a few local restaurants in each country), food costs can rise quite quickly. Going out for drinks with friends or locals, or "experiencing the nightlife" can also get quite expensive, though you often make up for it in other cities where you'll spend the day on the beach or window shopping and don't spend much at all.
This leaves you about €10 to €15 per day on odds and ends like metro/bus tickets, museum admissions, city tours, souvenirs and internet.
Please only use this as a very rough estimate - with a strict budget and careful spending, you should be able to get by with €60 a day and still have a fabulous time, but we don't recommend any less than this ... more is better - you don't have to spend it all, but it gives you more options. It's also a good idea to have a credit card or access to additional funds should the need arise. Credit cards can be invaluable as a backup plan or for emergencies - you don't want to end up in debt though.