$20-30 if you want to get something to eat, for a day trip to Faneuil Hall.
On a class field trip you will be busy and have little need for money. If you are to buy lunch you will be told before the trip. Other than that , save the money and stay out of temptation to spend it.
You will need at least 20-30$ luch will cost you almost 8$ and the rest could be for spending. Good Luck! :)