Since there is so much to include on a trip to any major city, the costs of the trip depend on the things that you want to include on your trip. It also depends upon where you are traveling from. With that in mind, it could cost a few thousand dollars.
A cheaper trip to Bangkok where you stay in cheaper hotels or hostiles and do more free sightseeing would be about $3,000 with a grand trip being close to $10,000 if you included the more expensive eating places and attractions. The bulk of the trip would most likely be the transportation. Flights cost a couple of thousand dollars based on when you decided to purchase your ticket. It also depends on the food you eat.