@victor198,
It all depends on you, your school's budget, and the grade of the students going.
Are you trying to set one up (as a teacher or school admin) for your school? Or are you a student whose looking into starting one for your school? Or are you a student whose school already has one and instead of asking them about it, you ask a group of strangers about the details? Every school trip is different. We can't provide the particulars (cost and itinerary).
I went to France in 1989 or 1990 for my high school trip. Paid it out of the money I saved up over the years of working a part time job doing dishwashing, a job I held throughout my four years in high school. We went to France for about two weeks. Spent most of that time in Paris. Spent a day in Nice and a day in Switzerland.