We fund $4 per child for field trips if the grade takes one field trip, or $6 per child if the grade takes two field trips. We also fund $750 per grade level for in-school activities, such as a dance troupe, storyteller, etc.
We give each class $250/year to spend on field trips. The teachers most often use it to pay for the busses. This year, we are giving the money in advance to the school secretary and she is going to manage it.
Most times, parents are asked to pay a few dollars to cover the cost of the field trip.
I've been on 5 PTAs in 3 different states - both public and charter schools. We have never funded field trips with the exception of an annual 6th grade weekend trip to an environment camp that every student did before graduation.