Start from the total, not the wishlist
Most trips go over budget for the same reason: the plan is built from desires and priced afterwards. Reverse it. Fix the total first, then let every decision compete for a share of it.
A useful baseline for a European city break is 30-35% transport, 25-30% accommodation, 15-20% food, 10-15% activities and a 5-8% reserve. The reserve is not optional — it is what stops one delayed train from wrecking the last two days.
Weight the split by the purpose of the trip
A romantic weekend spends more on stays and dining and less on activities. A backpacking month inverts that. Business travel concentrates on transport reliability and location, where a cheaper hotel forty minutes out costs more in taxis and lost hours than it saves.
This is exactly the logic Googjet applies automatically: the travel objective changes the allocation ratios before a single recommendation is shown.
Re-plan instead of overspending
When one line goes over, another must come down. Deciding that in advance — food before activities, shopping before stays — turns an overspend into a swap rather than a surprise.