The standard rate for a teachers as an hourly rate is £20ph. From my experience that is the expectated rate for supply etc.
At 1265 contact hours over a year, they earn £25000.
Therefore a teacher in their first few years of service is not being paid for 13 weeks holiday a year (it would be £33000 if they were paid for 52 weeks of contact time).
The teachers earning £33k a year are more experienced and have more responsibility (therefore would expect an increased hourly rate.
For the op - coach transport for schools may well be more (brownies I assume is evening or weekend?), subs would subsidise the trip,
Correct me if I'm wrong but doesn't the school trip cost also cover those who can't afford to pay it (I know that always starts a great debate!)