Europe is not one price. A month splitting time between Oslo and Lisbon costs wildly different amounts, so the first rule of budgeting Europe is: pick your countries to match your budget.
Cheap Europe (under $50/day): Portugal, Spain (south), Poland, Hungary, Romania, Bulgaria, Albania, much of the Balkans. Hostels $15-25, meals $8-12, beer under $3, intercity buses cheap.
Mid-range Europe ($50-80/day): Germany, Czech Republic, Italy (outside the big three), Greece, Croatia, Spain (north), Turkey. Hostels $25-40, a restaurant meal $15-25.
Expensive Europe ($80-150+/day): Scandinavia, Switzerland, UK, France (Paris), Amsterdam, Dublin. Hostels $35-60, a beer $8-10, a basic meal $25+.
The biggest costs are accommodation and intercity transport. Book hostels a few days ahead in high season. For trains, compare the rail pass against individual tickets — for long routes booked last-minute, point-to-point can be brutal; booked early, it's often cheaper than a pass. Buses (FlixBus, etc.) are the budget champion.
Eat like a student. Supermarket breakfasts, lunch deals, and one proper dinner keep food costs sane. In Italy, the aperitivo culture means a drink and a spread of free food for the price of a cocktail.
Set a daily budget per city in Wander Wallet and switch the currency as you cross borders. The trip budget rolls up across all of them, so you can see at a glance whether Lisbon's cheap days are funding your expensive swing through Switzerland.