I'm not sure if this fits the original situation, but we had a similar issue last fall.
We have multi-age classrooms in our school, where grades 4 and 5 are combined. The Music teacher wanted to take the 4th/5th grades on a field trip to the big theatre downtown for a live performance, and the total cost would be $1,000.
Since this year's 4th graders would be next year's 5th graders, the trip isn't necessary annually. Therefore, this year's 3rd graders would attend when they're in 5th grade. Are you with me still?
Since this was the first year, the PTO approved the $1,000 so this year's 5th graders could enjoy this wonderful experience, but we have worked out an annual budget of $500 for this expenditure.
Provided a child doesn't leave our school before 4th/5th grades, everyone will have the opportunity to go on this field trip.
Sheila
Co-Pres