It really just depends on the district. I know that we could never have carpooled because of liability issues. Some districts have money set aside for field trips, some don't. Some allow you to charge whatever, some don't. Some places you cannot charge anything--not even for the transportation.
We paid a flat field trip fee of $12 for the year when Eli was in pre-k. The only two field trips they took were to the nature center, which was free, and to the zoo, which we had to pay for. There were no buses. We had to take our own child and stay for the whole trip. So what was the $12 for? Paying the teachers for the extra time they spent on the trip (full day instead of half day). I should mention that ALL of the teachers had kids in the pre-k so they would have been their with their own child, anyhow.
I didn't read all the posts here, but we have a pumpkin/christmas tree farm that offers kids' play area with pumpkin ring toss game, crawl in corn ( like a sand box), jump in hay, and a farm animal petting zoo, pumpkin slinging, and hay rides. Their fee is 8.00 12 and over, and 5.00 age 3-11. I think your fee was just perfect.
