How are you going there? Car, Bus, Train, Flight?
Bus/Train/Flight – Just see the ticket price.
All the information is easily available on Google.
Transportation cost from Bus station/Train Station/ Airport to your Accommodation location and back.
Plus trips around the town. Are you going to use public transport? Are you going to use a taxi?
if more people are travelling – Taxi makes sense than public transport unless you are in place like Tokyo or New York.
There are sites like rome2rio which provides how much it will cost to get your trips.
What is cost of food in general at destination you are going to an average cost of meal at your favorite restaurant you going to dine is available on tripadvisor.
Are you going to see lot of sightseeing? Are you going to do lot of nightlife? Calculate cost per day.
You never know what you end up buying or you indulge yourself into something extra that you never planned. Maybe it’s a spa or may be cool thing you found on street side. Plan for it while budgeting. Better to come back with money from the trip then end up running completely dry at the destination.
If you are going to country where they don’t accept your currency, there would be a cost they would charge you convert the currency. There might be countries where you can’t technically bring the money back or they would be useless like Vietnam. Either you spend it all or change it before you get out. Unless you are planning to go back to the same place, you better get change before you reach the airport.
Forex at the airport is usually the worst prices. I have found western union places have the best conversion rates.
There is nothing more spoiler than being robbed, or flight not working or some injury while you were trekking or partying. Lot of times airlines lose your baggage in transit. Make sure you get your insurance before you go anywhere.
P.S The picture is of old town Dinan, France.