Most Europe budgets do not fail because someone forgot food. They fail in the seams between bookings: the airport forty kilometers from the city, the bag excluded from a fare, or the hotel tax due in cash at arrival.
The low-cost flight that is not low cost
Add baggage, seats if needed, priority rules, payment charges, and both airport transfers. Then check the departure time. A 06:00 flight may require a taxi or airport hotel when public transport has not started. Measure the journey from one hotel to the next.
Rail and ferry extras
Some trains require reservations beyond a pass or base fare. Night trains may price berths separately. Ferries can add seat, cabin, port, or baggage costs. Flexible tickets may be materially more expensive than the headline price shown in search results.
Accommodation charges
City or tourist taxes may be collected separately and vary by person, night, property category, or season. Apartments can add cleaning fees or deposits. Late check-in, resort facilities, parking, and breakfast can also sit outside the first nightly figure.
Money and connectivity
Dynamic currency conversion, out-of-network ATM fees, card foreign-transaction charges, and poor exchange counters can quietly compound. Decline conversion when your card issuer offers the better rate, and understand both the local ATM fee and your bank’s fee.
Roaming can be another surprise. Check the plan before departure and compare it with a local or regional eSIM where compatible. Keep access to booking details available offline.
The cost of an overpacked route
Every move creates local transfers, storage, meals in transit, and lost usable time. A fourth city can look cheap when only its hotel is counted. It may be expensive once the journey and shortened stays are included.
Build an omission check
Review the trip one day at a time and ask where you sleep, how you reach it, what must be reserved, and what is payable locally. Add a separate contingency after these costs are visible.
See What hidden costs should I expect on a Europe trip? and Are Europe city taxes included in hotel prices? before finalizing the full trip cost breakdown.