Not Kathy here, but I know that in 8th grade band I was jealous and disappointed to find out the band at the other middle school went 600 miles to Oregon for a multi-day band competition while our big trip was a one-day trip to the local university's music department.
From my adult perspective I know now that a trip like that was pretty expensive (and our little band probably wasn't good enough to compete).
Regardless, in the example Kathy gave I can imagine being disappointed and concluding that my art teacher couldn't be bothered to put together a trip.
After being on the parent side of the teacher-student-parent field trip equation for 10 years now, I think this kind of thing would be helped immensely if there was more communication between teachers and parents.
Teacher is over-loaded with work to do, doesn't have time to organize a field trip. Send out a letter to all the parents, ask if there are a couple of people willing to do some research and organization. Maybe PTA can donate some money, maybe the Rotary club can. If you can't ask $20 per student then find a cheaper venue. If no parent steps forward, fine, at least the teacher tried.
If you don't think the kids will get anything out of a trip to a museum then take them somewhere else, somewhere more engaging. Or better yet, confiscate the cell phones for the duration of the tour and have them do a scavenger hunt-type questionnaire.
I loved all the field trips I went on, every last one of them. That other students were bored was utterly baffling to me. Now I think it's sad.
Edited to add after reading the above, yes, if after various ideas have been tried and a field trip just isn't going to be possible then just say something. "Sorry guys, a field trip would be great but this year it's not going to happen because of this, this and this." As a parent I would appreciate and respect that.