We have 350 students and the PTO budgets to pay for all the field trip bus fees for the year ($2750 this year). Parents appreciate not being asked for the fee (yes, we are still allowed to ask, JHB), and the PTO feels we are providing a very tangible benefit to the school. In our district, we own our buses. If the field trip is within the school district, there is no cost to the school.
If the trip is close to the school, within a 1/2 hour and will be completed within the school day, our school uses the same company that has the city contract and we are able to get the bus at about $40. If it is a longer/larger trip that may run late due to traffic or whatever, we hire a bus from a local compant and these run $150 or more.
Each class in the school has a set amount to use for field trips, this yar each class gets $450. or each grade gets $900 to share. This money can go towards any part of the field trip. If any class spends more than that they have to take a collection.
Our school district does not have any buses, so the pto pays for a charter bus for all field trips (average cost is $150.00 per class for bus and $100.00 for admission for wherever they choose to go).
Our district owns the buses so for each field trip the individual school pays for the driver and gas. Most trips are completely funded by the principal from her general fund, but extras ones can be funded by the parents, (ie:4th grade is going to three symphonic concerts, parents paid admission, school pays driver and gas) we even send our 5th grade to our sister school in Mexico that involves an eleven hour yellow school bus ride!!YUCK!! PTO has never been asked to cover this cost because the money has always been there for it.
Our district's school bus operations are contracted out to a third party for normal day to day transport. (It's regular yellow school buses - but the operation is outsourced.) Field trips are extra and we use that same company. I'm don't believe we have a choice. Anyway, the school needs to budget to cover the cost of this transportation, which is one of the reasons so few field trips are planned. I think each class gets either 1 or 2 a year. I don't know if it's law or policy, but the school can not request that each student bring X amount ($1, $3, etc) for field trips. I believe 5th grade wanted to do something special, so they had some bakesales to raise the money.