A reliable trip estimate starts with the whole journey, not a daily average. List transport, lodging, food, local travel, activities, taxes, insurance, and a contingency separately. Estimate a low and high case for each category, then check the largest uncertainties against current supplier prices before booking.
Keep fixed commitments separate from flexible spending. This makes it clear which changes affect the route and which can be absorbed by the budget. Add a buffer for exchange-rate movement, baggage, transfers, and one unexpected night.