My sister teaches at a school that fundraises a lot. She goes on a field trip per month with her class. My friend teaches at an inner city school and she goes years between field trips. I asked her why when so many museums offer free admission for class groups and she said the cost of the bus and getting enough chaperones. I once organized the community outreach group at a place where I was working to go to the inner city school to volunteer. There was a class going on a field trip and each child had to provide their own bus fare for the city bus. I can't imagine the kids at my sister's school on public transportation, even if it were paid for them.